main/postmarketos-initramfs: umount /boot in the initramfs

Fixes a bug where the kernel thinks the boot partition is still mounted and it's unavailable for mounting in the rootfs

If you look at what the initramfs was doing before !5635 (merged), it was explicitly umounting the boot partition from /boot in the initramfs and then mounting it under /boot in the rootfs: https://gitlab.postmarketos.org/postmarketOS/pmaports/-/blob/c45d7ec0a5f7ac814b50087402bb11825120f42b/main/postmarketos-initramfs/init.sh#L87

With this patch, we're now back to umounting /boot in the initramfs. The second part of that original logic is now handled elsewhere now, and that seems to be working OK.

The bug was reported here: #3279 (closed) (comment 454346)