arm: Warn that starting with v2018.01 gcc-6 or later is required

There are more and more cases where if we do not use gcc-6.0 or later we
run into problems where our binaries are too large for the targets.
Given the prevalence of gcc-6.0 or later toolchains at this point in
time, we give notice now that starting with v2018.01 we will require
gcc-6 (or later) for ARM.
Signed-off-by: NTom Rini <trini@konsulko.com>

arch/arm/config.mk

@@ -45,7 +45,7 @@ endif
 
 # Only test once
 ifeq ($(CONFIG_$(SPL_)SYS_THUMB_BUILD),y)
-archprepare: checkthumb
+archprepare: checkthumb checkgcc6
 
 checkthumb:
 	@if test "$(call cc-name)" = "gcc" -a \
@@ -55,8 +55,18 @@ checkthumb:
 		echo '*** Your board is configured for THUMB mode.'; \
 		false; \
 	fi
+else
+archprepare: checkgcc6
 endif
 
+checkgcc6:
+	@if test "$(call cc-name)" = "gcc" -a \
+			"$(call cc-version)" -lt "0600"; then \
+		echo -n '*** Your GCC is older than 6.0 and will not be '; \
+		echo 'supported starting in v2018.01.'; \
+	fi
+
+
 # Try if EABI is supported, else fall back to old API,
 # i. e. for example:
 # - with ELDK 4.2 (EABI supported), use: