roms: Correct the EDK2_BASETOOLS_OPTFLAGS variable description

In commit 1cab4641 we incorrectly described the
EDK2_BASETOOLS_OPTFLAGS can pass CPPFLAGS and CFLAGS
options to the EDK2 build tools, but it only expands
the CFLAGS (not to the CPPFLAGS).
Update the description to be more accurate.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190506141923.12183-2-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>