configs: add defconfig for Freescale i.MX31 PDK

The i.MX31 Product Development Kit (or PDK) is Freescale development board
based on the i.MX31 application processor.

This defconfig is based on mainline kernel v3.15.10, and is aimed at a PDK in
"3 stack" configuration, with CPU engine board, personality board and debug
board.

Signed-off-by: Vincent Stehlé <vincent.stehle@freescale.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/freescale/imx31_3stack/readme.txt

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+**************************************
+Freescale i.MX31 PDK development board
+**************************************
+
+This file documents the Buildroot support for the Freescale i.MX31 PDK in "3
+stack" configuration.
+
+The i.MX31 Product Development Kit (or PDK) is Freescale development board [1]
+based on the i.MX31 application processor [2].
+
+For more details on the i.MX31 PDK board, refer to the User's Guide [3].
+
+Build
+=====
+
+First, configure Buildroot for your i.MX31 PDK board:
+
+  make freescale_imx31_3stack_defconfig
+
+Build all components:
+
+  make
+
+You will find in ./output/images/ the following files:
+  - rootfs.cpio
+  - rootfs.cpio.gz
+  - rootfs.tar
+  - zImage
+
+The generated zImage does include the rootfs.
+
+Boot the PDK board
+==================
+
+The i.MX31 PDK contains a RedBoot bootloader in flash, which can be used to
+boot the newly created Buildroot images from the network.
+
+This necessitates to setup a TFTP server first. This setup is explained for
+example in Freescale i.MX31 PDK 1.5 Linux User's Guide [4].
+
+Here is a sample RedBoot configuration, for proper network boot of Buildroot on
+the i.MX31 PDK:
+
+    RedBoot> fconfig -l
+    Run script at boot: true
+    Boot script:
+    .. load -r -b 0x100000 zImage
+    .. exec -c "console=ttymxc0,115200"
+
+    Boot script timeout (1000ms resolution): 2
+    Use BOOTP for network configuration: false
+    Gateway IP address: <your gateway IP address>
+    Local IP address: <your PDK IP address>
+    Local IP address mask: 255.255.255.0
+    Default server IP address: <your TFTP server IP address>
+    Board specifics: 0
+    Console baud rate: 115200
+    Set eth0 network hardware address [MAC]: false
+    GDB connection port: 9000
+    Force console for special debug messages: false
+    Network debug at boot time: false
+
+Adapt those settings to your network configuration by replacing the appropriate
+network addresses where necessary.
+
+You might want to verify that your i.MX31 PDK switches settings are the correct
+ones for UART, power, boot mode, etc. Here is a reference switches
+configuration:
+
+    SW4
+    1   2   3   4   5   6   7  8
+    ON off off off off off off ON
+
+    SW5 SW6 SW7 SW8 SW9 SW10
+     0   1   0   0   0    0
+
+See the i.MX31 PDK Linux Quick Start Guide [5] for more details on the switches
+settings.
+
+Connect a serial terminal set to 115200n8 and power on the i.MX31 PDK board.
+Buildroot will present a login prompt on the serial port.
+
+Enjoy!
+
+References
+==========
+
+[1] http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=i.MX31PDK
+[2] http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=i.MX31
+[3] http://cache.freescale.com/files/32bit/doc/user_guide/pdk15_imx31_Hardware_UG.pdf
+[4] http://cache.freescale.com/files/32bit/doc/support_info/IMX31_PDK15_LINUXDOCS_BUNDLE.zip, pdk15_imx31__Linux_UG.pdf
+[5] http://www.freescale.com/files/32bit/doc/quick_ref_guide/PDK14LINUXQUICKSTART.pdf

configs/freescale_imx31_3stack_defconfig

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+# architecture
+BR2_arm=y
+BR2_arm1136jf_s_r0=y
+BR2_ARM_EABIHF=y
+
+# toolchain
+BR2_KERNEL_HEADERS_VERSION=y
+# Note: sadly the Linux kernel will not boot on the i.MX31 PDK, starting with
+# v3.16 and at least up to v4.0-rc4; this is why we use v3.15.y here.
+BR2_DEFAULT_KERNEL_VERSION="3.15.10"
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_3_15=y
+
+# system
+BR2_TARGET_GENERIC_GETTY_PORT="ttymxc0"
+
+# kernel
+BR2_LINUX_KERNEL=y
+BR2_LINUX_KERNEL_SAME_AS_HEADERS=y
+BR2_LINUX_KERNEL_DEFCONFIG="imx_v6_v7"
+BR2_LINUX_KERNEL_ZIMAGE=y
+BR2_TARGET_ROOTFS_CPIO_GZIP=y
+BR2_TARGET_ROOTFS_INITRAMFS=y