Add kernel updater script (#625)

main/postmarketos-update-kernel/APKBUILD

+pkgname=postmarketos-update-kernel
+pkgver=0.0.1
+pkgrel=0
+pkgdesc="kernel updater script for postmarketOS"
+url="https://github.com/postmarketOS"
+depends="util-linux"
+source="update-kernel.sh"
+arch="noarch"
+license="GPL3"
+
+package() {
+	install -Dm755 "$srcdir/update-kernel.sh" \
+		"$pkgdir/sbin/pmos-update-kernel"
+}
+sha512sums="7d2f3031b1a468accff5a3584bd51d3607141b01e1d2a93d611852476bdeecc3edd7b80f8e3b034012d9b5f09de907af1de02f48586d82d06ee4b5746d4fd286  update-kernel.sh"

main/postmarketos-update-kernel/update-kernel.sh

+#!/bin/ash
+# shellcheck shell=dash
+set -e
+
+case $1 in
+	--help|-h|'')
+		echo "Usage: pmos-update-kernel [flavor]"
+		exit 1
+		;;
+esac
+
+# shellcheck disable=SC1091
+. /etc/deviceinfo
+
+FLAVOR=$1
+METHOD=${deviceinfo_flash_methods:?}
+case $METHOD in
+	fastboot|heimdall-bootimg)
+		BOOT_PARTITION=$(findfs PARTLABEL="boot")
+		echo "Flashing boot.img..."
+		dd if=/boot/boot.img-"$FLAVOR" of="$BOOT_PARTITION" bs=1M
+		;;
+	heimdall-isorec)
+		KERNEL_PARTITION=$(findfs PARTLABEL="${deviceinfo_flash_heimdall_partition_kernel:?}")
+		INITFS_PARTITION=$(findfs PARTLABEL="${deviceinfo_flash_heimdall_partition_initfs:?}")
+		echo "Flashing kernel..."
+		dd if=/boot/vmlinuz-"$FLAVOR" of="$KERNEL_PARTITION" bs=1M
+		echo "Flashing initramfs..."
+		gunzip -c /boot/initramfs-"$FLAVOR" | lzop | dd of="$INITFS_PARTITION" bs=1M
+		;;
+	0xFFFF)
+		echo -n "No need to use this utility, since uboot loads the kernel directly from"
+		echo " the boot partition. Your kernel should be updated already."
+		exit 1
+		;;
+	*)
+		echo "Devices with flash method: $METHOD are not supported."
+		exit 1
+		;;
+esac
+echo "Done."