main/postmarketos-initramfs: add support for multi-device btrfs

This change fixes booting with a multi-device btrfs volume as root filesystem. A multi-device btrfs volume will find all partitions or devices automatically when mounted, but this only works if btrfs device scan was run beforehand.

This adds a call to btrfs device scan before mounting if the root filesystem is btrfs.

The command will only run for btrfs root filesystems, so I consider this change fairly safe.

Fixes: #3617 (closed)

main/postmarketos-initramfs/APKBUILD

 # Maintainer: Caleb Connolly <caleb@postmarketos.org>
 # Co-Maintainer: Clayton Craft <clayton@craftyguy.net>
 pkgname=postmarketos-initramfs
-pkgver=3.8.0
+pkgver=3.8.1
 pkgrel=0
 pkgdesc="Base files for the postmarketOS initramfs / initramfs-extra"
 url="https://postmarketos.org"
@@ -109,7 +109,7 @@ sha512sums="
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+0581202eabe4edd4735d29b04f4da2a7a0fcce240a93d9e65d90d84e7e693b5ea28eec80b03783d1905a2e0ebfb5f2a266aa422525a5336eb8569fb37c2c7a33  init_functions.sh
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 675e7d5bee39b2df7d322117f8dcaccc274d61beaf4d50ead19bbf2109446d64b1c0aa0c5b4f9846eb6c1c403418f28f6364eff4537ba41120fbfcbc484b7da7  mdev.conf