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: NLaszlo Ersek <lersek@redhat.com>
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190506141923.12183-2-philmd@redhat.com>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

roms/Makefile

@@ -121,8 +121,8 @@ build-efi-roms: build-pxe-roms
 		$(patsubst %,bin-i386-efi/%.efidrv,$(pxerom_targets)) \
 		$(patsubst %,bin-x86_64-efi/%.efidrv,$(pxerom_targets))
 
-# Build scripts can pass compiler/linker flags to the EDK2 build tools
-# via the EDK2_BASETOOLS_OPTFLAGS (CPPFLAGS and CFLAGS) and
+# Build scripts can pass compiler/linker flags to the EDK2
+# build tools via the EDK2_BASETOOLS_OPTFLAGS (CFLAGS) and
 # EDK2_BASETOOLS_LDFLAGS (LDFLAGS) environment variables.
 #
 # Example: