device-generic-x86_64: add linux-postmarketos-mainline subpackage

Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7398>

device/community/device-generic-x86_64/APKBUILD

@@ -4,7 +4,7 @@ maintainer="Clayton Craft <clayton@craftyguy.net>"
 pkgname=device-generic-x86_64
 pkgdesc="Generic x86_64 system, with EFI"
 pkgver=22
-pkgrel=0
+pkgrel=1
 url="https://postmarketos.org"
 license="MIT"
 arch="x86_64"
@@ -26,6 +26,7 @@ source="
 install="$pkgname.post-install $pkgname.post-upgrade"
 subpackages="
 	$pkgname-intel
+	$pkgname-kernel-mainline:kernel_mainline
 	$pkgname-kernel-stable:kernel_stable
 	$pkgname-kernel-lts:kernel_lts
 "
@@ -56,6 +57,12 @@ intel() {
 		-t "$subpkgdir"/usr/share/mkinitfs/files/
 }
 
+kernel_mainline() {
+	pkgdesc="postmarketOS Mainline kernel"
+	depends="linux-postmarketos-mainline"
+	devicepkg_subpackage_kernel $startdir $pkgname $subpkgname
+}
+
 kernel_stable() {
 	pkgdesc="Alpine Stable kernel"
 	provides="device-tablet-x64uefi-kernel-edge=$pkgver-r$pkgrel device-tablet-x86uefi-kernel-edge=$pkgver-r$pkgrel $pkgname-kernel-edge=$pkgver-r$pkgrel"