lg-bullhead: adapt for mainline kernel support

Hi everyone, this is my first merge request to pmaports. (sorry if things are not quite the way they're meant to be)

I'm continuing the work of @pevik on the support of MSM8992/MSM8994 devices by adding the missing code for mainline support of the Nexus 5x.

I haven't been able to test this on my device (due to the eMMC issue), hence the Draft status of this MR.

device/testing/device-lg-bullhead/APKBUILD

 # Reference: <https://postmarketos.org/devicepkg>
 pkgname=device-lg-bullhead
 pkgdesc="Google Nexus 5X"
-pkgver=0.1
-pkgrel=13
+pkgver=1
+pkgrel=0
 url="https://postmarketos.org"
 license="MIT"
 arch="aarch64"
-options="!check !archcheck"
-depends="postmarketos-base linux-lg-bullhead mkbootimg mesa-dri-swrast msm-fb-refresher"
+depends="
+	mesa-dri-gallium
+	mkbootimg
+	msm-fb-refresher
+	postmarketos-base
+	postmarketos-update-kernel
+"
 makedepends="devicepkg-dev"
+subpackages="
+	$pkgname-kernel-downstream:kernel_downstream
+	$pkgname-kernel-mainline:kernel_mainline
+	$pkgname-nonfree-firmware:nonfree_firmware
+"
 source="deviceinfo"
-subpackages="$pkgname-nonfree-firmware:nonfree_firmware"
+options="!check !archcheck"
+
+build() {
+	devicepkg_build $startdir $pkgname
+}
+
+kernel_downstream() {
+	pkgdesc="Downstream kernel"
+	depends="linux-lg-bullhead"
+	devicepkg_subpackage_kernel $startdir $pkgname $subpkgname
+}
+
+kernel_mainline() {
+	pkgdesc="Close to mainline kernel (only basic hardware support)"
+	depends="linux-postmarketos-qcom-msm8994"
+	devicepkg_subpackage_kernel $startdir $pkgname $subpkgname
+}
 
 nonfree_firmware() {
 	pkgdesc="Proprietary WiFi, Modem, GPU and sensors firmware"
@@ -18,15 +44,10 @@ nonfree_firmware() {
 	mkdir "$subpkgdir"
 }
 
-
-build() {
-	devicepkg_build $startdir $pkgname
-}
-
 package() {
 	devicepkg_package $startdir $pkgname
 }
 
 sha512sums="
-e98b8a6f31dd5ba6cad8940771a5e5c7d0ff6e236ba79f52083f012292a6becea1452f7a53d06b7a1ed7ac1bf3187e994aeae1d6315dcdb757af4578c3c41173  deviceinfo
+a70681efe845eb0ca8aaaf935408e283c7a4b95adadff49327fecfc6dcdba8d0ef4cf7ca98c21580c1f3c2c4346366cd86108f0bba08a3bf923d1a02238bc2f9  deviceinfo
 "