pmbootstrap install: support new ondev boot label

In order to support FAT32 as boot partition, the label is shortened from pmOS_inst_boot to pmOS_i_boot in pmaports!2337. Read the value from pmaports.cfg and fall back to the old value, so both are supported (-> building v21.06 and v21.03 will use the old label).

Remove the old codepath that would set "pmOS_boot" as label for the install partition, if the postmarketos-ondev package was older than 0.4.0. This is only the case on the long unsupported v20.05 branch.

Fixes #2041 (closed)

(Order of merging doesn't really matter since it's all backwards compatible, but I've added the "fixes" here, so let's merge the pmaports MR first and then this one.)