rescue-mode-omnia: set the bootable flag on created mmc partition

I have a dream: using a completely upstream u-boot on my Turris Omnia one day :-)

The standard u-boot distribution looks for the bootable flag on partitions to determine whether it should try to boot from there. Adding the bootable flag when reflashing a medkit seems like a very low risk and easy change: just add an "a" before the "p" here: https://gitlab.labs.nic.cz/turris/turris-os-packages/blob/ef0c02b7f39c23c1ebffc6f04db1e7edd47cfb2e/cznic/rescue-mode-omnia/files/rescue.sh#L130

I would have sent a PR but can't figure out how to do that on your gitlab. Could someone take care of this? Any objections?