uuu_scripts: Add helper to flash MBR with resized root partion

This makes sure the root partition extends to the full MMC.

See Librem5/librem5-devkit-tools!226

Closes: https://source.puri.sm/Librem5/Apps_Issues/-/issues/104
4 jobs for emmc-mbr in 2 minutes and 25 seconds (queued for 31 seconds)
Status Job ID Name Coverage
  Package
passed #364443
librem5
package-pureos-byzantium

00:00:51

passed #364444
librem5:arm64
package-pureos-byzantium:arm64

00:02:20

 
  Test Package
passed #364445
librem5
autopkgtest-pureos-byzantium-package

00:00:54

passed #364446
librem5
lintian-pureos-byzantium-package

00:00:47