u-boot: add an option to indicate that DTC is needed

Some U-Boot configurations require the Device Tree compiler to be
available, so we need to depend on host-dtc (example configuration:
zynq_zed).

However, we don't want to build it unconditionally, since the vast
majority of U-Boot configurations don't need it (and host-dtc itself
has a bunch of dependencies).

So, we simply add a Config.in option that allows users to indicate
whether their U-Boot needs DTC or not, and depend on host-dtc if this
option is enabled.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/uboot/Config.in

@@ -139,6 +139,13 @@ config BR2_TARGET_UBOOT_CUSTOM_CONFIG_FILE
 	  Path to the U-Boot configuration file.
 endif
 
+config BR2_TARGET_UBOOT_NEEDS_DTC
+	bool "U-Boot needs dtc"
+	select BR2_PACKAGE_HOST_DTC
+	help
+	  Select this option if your U-Boot board configuration
+	  requires the Device Tree compiler to be available.
+
 choice
 	prompt "U-Boot binary format"
 	default BR2_TARGET_UBOOT_FORMAT_BIN

boot/uboot/uboot.mk

@@ -77,6 +77,10 @@ UBOOT_MAKE_OPTS += \
 	CROSS_COMPILE="$(TARGET_CROSS)" \
 	ARCH=$(UBOOT_ARCH)
 
+ifeq ($(BR2_TARGET_UBOOT_NEEDS_DTC),y)
+UBOOT_DEPENDENCIES += host-dtc
+endif
+
 # Helper function to fill the U-Boot config.h file.
 # Argument 1: option name
 # Argument 2: option value