Makefile: don't use CFLAGS for environment text file

We use KCPPFLAGS to let the user set flags when invoking the C precompiler.
These should also be used when generating the environment text file.

Reported-by: Dave Jones <dave.jones@canonical.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

Makefile

@@ -1839,7 +1839,7 @@ ENV_FILE := $(if $(ENV_SOURCE_FILE),$(ENV_FILE_CFG),$(wildcard $(ENV_FILE_BOARD)
 quiet_cmd_gen_envp = ENVP    $@
       cmd_gen_envp = \
 	if [ -s "$(ENV_FILE)" ]; then \
-		$(CPP) -P $(CFLAGS) -x assembler-with-cpp -undef \
+		$(CPP) -P $(cpp_flags) -x assembler-with-cpp -undef \
 			-D__ASSEMBLY__ \
 			-D__UBOOT_CONFIG__ \
 			-I . -I include -I $(srctree)/include \