Remove compiler warning: target CPU does not support interworking

This warning is issued by modern ARM-EABI GCC on non-thumb targets.

Signed-off-by: Vladimir Panfilov <pvr@emcraft.com>
Signed-off-by: Sergei Poselenov <sposelenov@emcraft.com>
diff --git a/cpu/arm720t/config.mk b/cpu/arm720t/config.mk
index 641b91c..3cae1dc 100644
--- a/cpu/arm720t/config.mk
+++ b/cpu/arm720t/config.mk
@@ -32,4 +32,5 @@
 #
 # =========================================================================
 PLATFORM_CPPFLAGS +=$(call cc-option,-mapcs-32,-mabi=apcs-gnu)
+PLATFORM_CPPFLAGS +=$(call cc-option,-mno-thumb-interwork,)
 PLATFORM_RELFLAGS +=$(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,))