board/chromebook-snow: force checking hashes

The chromebook-snow config uses a custom kernel version, so add a hash
for it.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Alex Suykov <alex.suykov@gmail.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

board/chromebook/snow/patches/linux-headers/linux-headers.hash

@@ -0,0 +1 @@
+../linux/linux.hash

board/chromebook/snow/patches/linux/linux.hash

@@ -0,0 +1,2 @@
+# Localy computed
+sha256  5a26478906d5005f4f809402e981518d2b8844949199f60c4b6e1f986ca2a769  linux-4.15.tar.xz

configs/chromebook_snow_defconfig

@@ -2,6 +2,7 @@ BR2_arm=y
 BR2_cortex_a15=y
 BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_15=y
 BR2_GLOBAL_PATCH_DIR="board/chromebook/snow/patches"
+BR2_DOWNLOAD_FORCE_CHECK_HASHES=y
 BR2_TARGET_GENERIC_GETTY_PORT="tty1"
 BR2_TARGET_GENERIC_GETTY_TERM="linux"
 BR2_ROOTFS_POST_BUILD_SCRIPT="board/chromebook/snow/sign.sh"