kbuild: add user-supplied CPPFLAGS, AFLAGS and CFLAGS

Like Linux Kernel, this commit provides environment variables
KCPPFLAGS, KAFLAGS and KCFLAGS, which are useful to pass
additional options.
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

Makefile

@@ -584,6 +584,11 @@ endif
 
 export CONFIG_SYS_TEXT_BASE
 
+# Add user supplied CPPFLAGS, AFLAGS and CFLAGS as the last assignments
+KBUILD_CPPFLAGS += $(KCPPFLAGS)
+KBUILD_AFLAGS += $(KAFLAGS)
+KBUILD_CFLAGS += $(KCFLAGS)
+
 # Use UBOOTINCLUDE when you must reference the include/ directory.
 # Needed to be compatible with the O= option
 UBOOTINCLUDE    := \