pmbootstrap install: support new ondev boot label (MR 2083)

In order to support FAT32 as boot partition, the label is shortened from
pmOS_inst_boot to pmOS_i_boot. 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.