Move dtbs to /boot

When the first kernels with seperate dtbs were added we decided to put the compiled dtb files in /usr/share/dtbs. Alpine puts their dtbs in /boot/dtbs though so instead of the mkinitfs copying the right dtb for the device into /boot or appending it to the boot.img the dtbs will just already be in /boot.

For u-boot devices this will change that uboot can selects its own dtb from /dtbs/$dtbname (because /boot is / in the uboot script).

For boot.img devices we don't really have /boot being a real /boot so it's just adjusting mkinitfs to append the dtb from the new path and nothing will change.

This change should make it easier to use alpine kernels instead of postmarketOS kernels for devices. It should even make it possible to have a generic aarch64 image that can boot on arm systems that are supported upstream and folling the arm uefi booting standard.

Also we currently strip the folder name from dtbs causing allwinner/sun50i-a64-pinephone-1.2.dtb to not have allwinner in the path. This causes hacks like these when we actually want to use the $dtbname: https://gitlab.postmarketos.org/postmarketos/pmaports/-/blob/f7d743a2138085d773df8731a34f59d258ed9b47/device/community/device-pine64-pinephone/APKBUILD#L45