main/postmarketos-mkinitfs: fix space issues

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.