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.