Drop our local sstrip copy and use the current ELFKickers upstream
version.

Patch the original makefile in order to avoid building elftoc, since it
fails with musl's elf.h. This is fine, since we only need sstrip anyway.

Finally, add the possibility to pass additional arguments to sstrip and
pass -z (remove trailing zeros) by default, which matches the behaviour
of the previous version.

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>
[shorten long commit msg lines]
Signed-off-by: Paul Spooren <mail@aparcar.org>