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)

main/postmarketos-initramfs/APKBUILD

 # Maintainer: Caleb Connolly <caleb@postmarketos.org>
 # Co-Maintainer: Clayton Craft <clayton@craftyguy.net>
 pkgname=postmarketos-initramfs
-pkgver=3.4.0
-pkgrel=1
+pkgver=3.4.1
+pkgrel=0
 pkgdesc="Base files for the postmarketOS initramfs / initramfs-extra"
 url="https://postmarketos.org"
 options="!check"  # no tests
@@ -105,7 +105,7 @@ sha512sums="
 9c0e8f6f61d5da191e03a1aa9d5d0ceb5baf1eae6dbb9bfb0af59817783525119ac8394b135f303f7b6434a3eab0b49185fb90379e06823db847a4999c75ce33  00-initramfs-base.dirs
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-5dc7d65e3e6f0df07d2f7ba661dfb6714a3c3bae6a9845a6d116582780ea69e34ba5950e30fc8dd0a4852e8f2b8b1ae9ff2ea962c4c4e4b34df5e3c30d8bbb3d  init.sh
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 18a0ca97bca094aeb8cb1c2582205ac0aff131085c39f1715246541cacfb35434aba515cd5d338703843efb36b9effd494a51361c51fb39be6052ad866346d3d  init_2nd.sh
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