arm: clang: Update support slightly

- Move most of the flags required into LLVM_RELFLAGS to test at build
  time instead of requiring them to be passed in.
- Update doc/README.clang to reflect this
- Switch to rpi_2 as the example as it's closer to working out of the
  box than rpi is.

Cc: Jeroen Hofstee <jeroen@myspectrum.nl>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/config.mk b/arch/arm/config.mk
index 9af6c37..9a5a974 100644
--- a/arch/arm/config.mk
+++ b/arch/arm/config.mk
@@ -19,6 +19,12 @@
 PLATFORM_RELFLAGS += $(call cc-option, -msoft-float) \
       $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,))
 
+# LLVM support
+LLVMS_RELFLAGS		:= $(call cc-option,-mllvm,) \
+			$(call cc-option,-target arm-none-eabi,) \
+			$(call cc-option,-arm-use-movt=0,)
+PLATFORM_RELFLAGS	+= $(LLVM_RELFLAGS)
+
 PLATFORM_CPPFLAGS += -D__ARM__
 
 # Choose between ARM/Thumb instruction sets