linux: reword binutils 2.29+ comment

This issue only applies to kernels built with CONFIG_THUMB2=y, so reword the
comment to make that more clear.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

linux/Config.in

@@ -8,7 +8,7 @@ config BR2_LINUX_KERNEL
 
 if BR2_LINUX_KERNEL
 
-comment "Linux kernel may fail to boot with binutils >= 2.29"
+comment "Linux kernel in thumb mode may be broken with binutils >= 2.29"
 	depends on BR2_arm || BR2_armeb
 	depends on !BR2_BINUTILS_VERSION_2_28_X