arm: Migrate SYS_THUMB_BUILD to Kconfig, introduce SPL_SYS_THUMB_BUILD

Today, we have cases where we wish to build all of U-Boot in Thumb2 mode for
various reasons.  We also have cases where we only build SPL in Thumb2 mode due
to size constraints and wish to build the rest of the system in ARM mode.  So
in this migration we introduce a new symbol as well, SPL_SYS_THUMB_BUILD to
control if we build everything or just SPL (or in theory, just U-Boot) in
Thumb2 mode.

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile
index 166fa9e..b95e105 100644
--- a/arch/arm/lib/Makefile
+++ b/arch/arm/lib/Makefile
@@ -71,7 +71,7 @@
 
 # some files can only build in ARM or THUMB2, not THUMB1
 
-ifdef CONFIG_SYS_THUMB_BUILD
+ifdef CONFIG_$(SPL_)SYS_THUMB_BUILD
 asflags-$(CONFIG_HAS_THUMB2) += -DCONFIG_THUMB2_KERNEL
 ifndef CONFIG_HAS_THUMB2