main/u-boot: add support for the HiFive Unmatched

The HiFive Unmatched is a riscv64 development plattform developed by
SiFive. The HiFive Unmatched has several boot loader stages which are
described further in Chapter 6 of the FU740-C000 manual. From these boot
loader stages Alpine needs to provide u-boot SPL and u-boot.itb. The
former initializes hardware for the first stages of the boot flow, the
later encapsulates OpenSBIs fw_dynamic payload, u-boot-nodtb.bin, and
the device tree blob for the HiFive Unmatched. As such, OpenSBI also
needs to be moved to main/ and is a makedependency of the u-boot package
on riscv64. Furthermore, updates of the OpenSBI package now require
rebuilds of the u-boot package on riscv64.

The packaged u-boot bootloader stages can either be loaded from an SD
card or from SPI flash. For Alpine purposes the SPI flash usecase is
probably more interesting but hasn't been integrated with the
update-u-boot script yet.

Currently, the files shipped by the u-boot-unmatched package are
primarily useful to (manually) create bootable SD card images.

See https://u-boot.readthedocs.io/en/latest/board/sifive/unmatched.html

main/u-boot/APKBUILD

@@ -3,7 +3,7 @@
 # Maintainer: Milan P. Stanić <mps@arvanta.net>
 pkgname=u-boot
 pkgver=2021.07
-pkgrel=0
+pkgrel=1
 pkgdesc="u-boot bootloader common files"
 url="https://www.denx.de/wiki/U-Boot/"
 arch="all !s390x !ppc64le"
@@ -20,6 +20,11 @@ source="https://ftp.denx.de/pub/u-boot/u-boot-${pkgver//_/-}.tar.bz2
 	"
 builddir="$srcdir"/u-boot-${pkgver//_/-}
 
+if [ "$CARCH" = "riscv64" ]; then
+	makedepends="$makedepends opensbi"
+	export OPENSBI=/usr/share/opensbi/generic/firmware/fw_dynamic.bin
+fi
+
 # secfixes:
 #   2021.04-r0:
 #     - CVE-2021-27097
@@ -47,6 +52,7 @@ aarch64) board_configs="
 	";;
 riscv64) board_configs="
 	qemu:qemu-riscv64
+	unmatched:sifive_unmatched
 ";;
 esac
 
@@ -136,8 +142,14 @@ _split_boards() {
 		msg "Including board $board"
 		mkdir -p "$subpkgdir"/usr/share/$pkgname/$board
 		export BUILD_DIR="$builddir"/build/$board
+
+		local board_images=""
+		case "$board" in
+			"sifive_unmatched") board_images="u-boot.itb spl/u-boot-spl.bin --" ;;
+		esac
+
 		local ok=no
-		for image in u-boot-sunxi-with-spl.bin -- MLO SPL u-boot.img -- u-boot.bin; do
+		for image in $board_images u-boot-sunxi-with-spl.bin -- MLO SPL u-boot.img -- u-boot.bin; do
 			if [ "$image" = "--" ]; then
 				[ "$ok" = yes ] && break
 				continue