Makefile: Fix project_filelist.txt generation

Makefile: Fix project_filelist.txt generation

The ctags tool (called by ctags-project target) currently complains
about not finding certain files.

The project_filelist.txt generation includes the compiler
generated "*.d" files, except for files found in build/util. Most file
paths in these "*.d" files are file paths relative to the root
directory of coreboot. Some projects though are compiled separately from
coreboot (e.g. payload, vboot, util). Some of these (e.g. util, vboot)
are also put into the build directory of coreboot and relative file
paths are relative to these projects instead of coreboot. This has the
uncanning side effect that the ctags Makefile target can't find these
files, since they are not relative to the coreboot root directory.

This patch also excludes the build/external directory from those files,
since they contain 'separately' compiled projects like 3rdparty/vboot.
That fixes the ctags-project Makefile target.

Change-Id: I16294171c29a0d5fd25a31018846f1013e130ee0
Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/71517
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>