cmd: mvebu/bubt: Add support for selecting eMMC HW partition

Support for burning into the correct eMMC HW boot partition was broken and
removed in commit 96be2f07 ("mvebu: bubt: Drop dead code"). Reimplement
this functionality and bring it back again.

Fixes: 96be2f07 ("mvebu: bubt: Drop dead code")
Signed-off-by: Pali Rohár <pali@kernel.org>