Add additional files into initramfs when debug-shell is installed

This PR uses the same approach as the Alpine's mkinitfs (also discussed in https://github.com/postmarketOS/pmbootstrap/issues/1188#issuecomment-363897670) to add the possibility to the hooks to include additional binaries to the initramfs.

The mkinitfs script search the *.files in the path /etc/postmarketos-mkinitfs/files/ and each line of those files is a binary file that has to be added to the initramfs.

Right now only the debug-shell hook uses this approach to add some files mentioned in #785 that are useful for the kernel mainlining process.

aports/main/postmarketos-mkinitfs/APKBUILD

 pkgname=postmarketos-mkinitfs
-pkgver=0.6.1
+pkgver=0.6.2
 pkgrel=0
 pkgdesc="Tool to generate initramfs images for postmarketOS"
 url="https://github.com/postmarketOS"
@@ -25,4 +25,4 @@ package() {
 }
 sha512sums="a09a48026a5cac308fa54e37c72e89fd7901e749f73720f3e8d3ec11a4d1527dbc1e5011aff53b1beffb6ff444c8df11a56e43d27ac0180ce8ddc79332be1d0d  init.sh.in
 f45548b295393cb078e4c456acef0eaac11705c4b5bc33981c320580c843231589fdfc78b25e182cd202435381d1be37085df01b6e272eee0af2ca0f89b3b4f5  init_functions.sh
-5075df824ae86cc73590513a2212411a5aa2a840268c3850ea22e541717ce254e15b812f98c22e708fb0a21d87b8c6eb0059cf667ca3350412e39dad2d81879e  mkinitfs.sh"
+833f8a6c3753da7581fefb56a6a68f61663f2910d6b89b7a9cc3033ac922589b1d07ba246cbcd8e0a99fc8df128f9ae1b1bf2bd8cf3cad65a323cf712c03ae2d  mkinitfs.sh"