Handle vexpress board removal in U-Boot

Looks like the board we use, vexpress_ca9x4, was removed earlier this year in U-Boot. This will be a hard blocker for us to update for us to update to the next Yocto LTS in April 2022.

Estimating this is tricky, because it's not clear what the fix will be. Some alternatives I can think of are:

• vexpress_ca9x4 was added more than five years ago. There may be better alternatives today. Check if a more recent board can be booted through QEMU and used in testing.
• Do what the commit message says: Adapt the board to the CONFIG_DM_MMC framework and reintroduce it upstream. This is an ancient board, probably not in use anymore, but they may be sympathetic to the cause if it means U-Boot can be tested in QEMU.

Progress

• Pull request: https://github.com/mendersoftware/meta-mender/pull/1456
• Need to revert and submit to meta-mender/master-next in order for reverts to be picked up in MEN-3915.
• Create ticket for upstream submission: MEN-5023