Enable RETPOLINE & KASLR

When `makepkg --nobuild --force' fails there is no reason to try to run
`make oldconfig'.

Currently extra architectures files (Kconfig mainly) was not removed because
the removal occurs before the copy.

- add deps
- shorter replacement line

Do not remove .git directory because it cause successive call to makepkg to
not work. Use .scmversion file instead.

- rely on kernel-install-hook package (no more call to kernel-install)
- run depmod inside the package to keep tracks of files
- fix patch application loop
- use pkgrel as kernel build version
- fix call to strip
- remove optdeps (they are in kernel-install-hook)