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>