ARM: enable Thumb build

Enable Thumb build and ARM-Thumb interworking based on the new
config flag CONFIG_SYS_THUMB_BUILD

Signed-off-by: Aneesh V <aneesh@ti.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/README b/README
index 79016e6..6919392 100644
--- a/README
+++ b/README
@@ -432,6 +432,14 @@
 		Select high exception vectors of the ARM core, e.g., do not
 		clear the V bit of the c1 register of CP15.
 
+		CONFIG_SYS_THUMB_BUILD
+
+		Use this flag to build U-Boot using the Thumb instruction
+		set for ARM architectures. Thumb instruction set provides
+		better code density. For ARM architectures that support
+		Thumb2 this flag will result in Thumb2 code generated by
+		GCC.
+
 - Linux Kernel Interface:
 		CONFIG_CLOCKS_IN_MHZ