bsdinstall: add warning when unsupported partition is modified

Right now is possible to modify bootable partition type to
non-bootable type without getting warning from partedit.
Example: if you auto parition drive for arm64, you will
get freebsd-ufs as bootable partition; now you are able
to change bootable partition type to freebsd-zfs; there
will be no warning and the system will install but
will not be bootable afterwards.

After this fix, partedit will issue the same warning it
does when user attempts to create bootable partition of
not supported type, notyfing a user about incoming
problem and allowing to think the decision over before
commiting the schema.

This has been tested on amd64 and arm64.

Obtained from: Semihalf
Submitted by: Dominik Ermel <der@semihalf.com>
Sponsored by: Cavium
Reviewed by: nwhitehorn
Differential Revision: https://reviews.freebsd.org/D6879