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/configs/twister_defconfig b/configs/twister_defconfig
index fefd490..9928d0e 100644
--- a/configs/twister_defconfig
+++ b/configs/twister_defconfig
@@ -1,4 +1,5 @@
 CONFIG_ARM=y
+# CONFIG_SYS_THUMB_BUILD is not set
 CONFIG_OMAP34XX=y
 CONFIG_TARGET_TWISTER=y
 CONFIG_FIT=y