boot/at91dataflashboot: force arm mode instead of Thumb mode

The at91dataflashboot code contains some hand-written ARM assembly
that uses ARM classic instructions, and will not build in Thumb-1
mode.

This issue has always existed in Buildroot, but it's only since we
started testing random configurations, including Thumb-1
configurations, that this issue popped up.

Fixes:

  http://autobuild.buildroot.net/results/68bf2291201ef1882d8e8d3eca2b1a5ad731e4b0/

Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 07acc00add7ac09114f3c01b85d794a31c68229c)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/at91dataflashboot/at91dataflashboot.mk

@@ -11,9 +11,14 @@ AT91DATAFLASHBOOT_SITE = ftp://www.at91.com/pub/buildroot
 AT91DATAFLASHBOOT_INSTALL_TARGET = NO
 AT91DATAFLASHBOOT_INSTALL_IMAGES = YES
 
+AT91DATAFLASHBOOT_CFLAGS = $(TARGET_CFLAGS) -fno-stack-protector
+ifeq ($(BR2_ARM_INSTRUCTIONS_THUMB),y)
+AT91DATAFLASHBOOT_CFLAGS += -marm
+endif
+
 define AT91DATAFLASHBOOT_BUILD_CMDS
 	make -C $(@D) CROSS_COMPILE=$(TARGET_CROSS) \
-		CFLAGS="$(TARGET_CFLAGS) -fno-stack-protector"
+		CFLAGS="$(AT91DATAFLASHBOOT_CFLAGS)"
 endef
 
 define AT91DATAFLASHBOOT_INSTALL_IMAGES_CMDS