[PPC] Fix loader input with newer QEMU versions

At least since version 4.0.0, QEMU became bug-compatible with PowerVM's
vty, by inserting a \0 after every \r. As this confuses loader's
interpreter and as a \0 coming from the console doesn't seem reasonable,
it's now being filtered at OFW console input.

Reviewed by: jhibbits
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20676