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)

device/testing/linux-alcatel-pop7lte/APKBUILD

@@ -40,7 +40,7 @@ builddir="$srcdir/$_repository-$_commit"
 
 prepare() {
 	default_prepare
-	downstreamkernel_prepare "$srcdir" "$builddir" "$_config" "$_carch" "$HOSTCC"
+	. downstreamkernel_prepare
 }
 
 build() {