main/postmarketos-mkinitfs: fix space issues (MR 2051)

In order to have hardware acceleration in osk-sdl, we have included mesa
for some devices (like the PinePhone) in initfs-extra. Mesa drags in
libLLVM, which got a size increase of 27 MB in the last version. This
leads to running out of space on the boot partition:

  ==> initramfs: creating /boot/initramfs-postmarketos-allwinner-extra
  gzip: write error: No space left on device
  cpio: write error: Broken pipe

Improve the situation by not directly writing initfs-extra to /boot
(next to the old version), but to a temporary directory first. Replace
the old version with the new version afterwards (like before).

This is a rather minimal fix for this specific issue. The mkinitfs
script needs more fixing (it's one of the oldest pieces of code in
postmarketOS and could use a rewrite...), but that's out of scope for
this patch.