Makefile: update clean-symlink target

This almost completely replaces the original clean-symlink target to
remove links from site-local into the coreboot tree. Changes include:

- Symbolic links removed are based on the EXTERNAL_SYMLINKS value of
symlink.txt files under site-local.
- Verify that there are site-local symlink.txt files to work on before
doing anything.
- Verify that the symlink.txt files reference links inside the coreboot
directory.
- Print out whether or not there are remaining symbolic links in the
tree.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: Ife0e7cf1b856b7394cd5e1de9b35856bd984663c
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83124
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <ericllai@google.com>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

0 files changed, 0 insertions, 0 deletions