devicepkg-dev: implement out-of-tree build for downstream kernels

Closes #493 (closed) and depends on pmbootstrap!1893 (merged)

Kernels that do not compile:

• linux-motorola-athene: doesn't compile at least since !976 (closed)
• linux-lg-bullhead: doesn't compile at least since !976 (closed)

main/devicepkg-dev/APKBUILD

 pkgname="devicepkg-dev"
-pkgver=0.12.0
+pkgver=0.12.1
 pkgrel=0
 pkgdesc="Provides default device package functions"
 url="https://postmarketos.org"
@@ -63,8 +63,8 @@ package() {
 sha512sums="d69930dd790b00fb39760a37d95a10899f0d167e10e2804feb05d9ce04f94185dc32d36edc90214aba2ea2aa09bf18f7dab93f1d2eff23f67beb2cc83be30e7c  compiler-gcc.h
 332f693abfb3260ad6c1ef5d245062af8cc8eedba7db360d54db5a7fcf7b301a37a299a0004e9ff4de80b5dfcd260d0cd0f99f96ed6d56080bb4dea81116cc82  devicepkg_build.sh
 10bda1a999891d6234bcfa3eb277b125c02361a031c5ff8445dabaae11e289321a6227f9f0973c7aa143a3df0512399bd0e3b2aa5a8ee560b28e1b135bad8635  devicepkg_package.sh
-69dad4bde05377ec235d72789c80cdfbdaa5a19b97defe9d4c19e0e12aa2c081d6dcf19fb80ebe33c2d6539ce8a39d36d98413404852d6bb2d56cc75d2f1ee95  downstreamkernel_prepare.sh
-8a6a0afa6e499b59c5433194298d13eb07712c4344e1dd4f71fdc34a2836950a12247c61aa7563bd646efff6118b377ceb387f09170282896809a160b3d9cd04  downstreamkernel_package.sh
+f7583b92a27b39d79762700a48853b8d641845cdd45eb875d6fddc236f4d70598927d4dfc7d237870617b77b42820686692a68220f610dbc01c76402d9edfb2e  downstreamkernel_prepare.sh
+776d7c36afc7bedbd531641abe90fa4bf06a519ad9e300dedf67b6d3b4997b57a398938dd528d5eeda6d17e0d34644472702a4fdd8ff41e0a4a7ad0671f4a216  downstreamkernel_package.sh
 cf5ee240cd1c1e9d30cdec833b4a007fd2e00f9a32ba3f265f99aa2e3dd3601cf43c08d3f3e01bade1d5b2648a6754b2f236e5cb4a9945e18e5c4e97aa2ed7c8  devicepkg_subpackage_kernel.sh
 9bb7f2a0930f397a713e9f4b6d5b83a426d9a2a3f692dcc42ac30717bf26ead869d8823a38f3ad388af12b2b9a02e8ec4d4418e9c2062389ed06d2b891a49ff3  deviceinfo
 136247a16ec91dc0c7241eeddb28c2196ae3b29946a9bc7e9566f848491ef1c53b12d05bf2dbc1cc352986712fd76f25c1510bcc8f301af540a2f01c33b299e1  expected-deviceinfo-downstream