device/device-raspberry-pi: fix boot error caused by missing dtb file (!870)

The upstream linux-rpi package has been modified: the dtb files
are now located in /boot/dtbs.

device/device-raspberry-pi/APKBUILD

@@ -2,7 +2,7 @@
 pkgname="device-raspberry-pi"
 pkgdesc="Raspberry Pi"
 pkgver=0.1
-pkgrel=6
+pkgrel=7
 url="https://postmarketos.org"
 license="MIT"
 arch="armhf"

device/device-raspberry-pi/device-raspberry-pi-kernel-rpi.post-install

@@ -1,5 +1,4 @@
 #!/bin/sh
 
-kernver=$(cat /usr/share/kernel/rpi/kernel.release)
-cd /usr/lib/linux-${kernver}/
+cd /boot/dtbs-rpi
 find . -type f -regex ".*\.dtbo\?$" -exec install -Dm644 {} /boot/{} \;