init: fix bug where environment vars can't be passed via boot args

Commit 026cee00 had the side-effect of dropping the '=' from
the unknown boot arguments that are passed to init as environment
variables.  This is because parse_args() puts a NUL in the string
where the '=' was when it passes the "param" and "val" pointers
to the parsing subfunctions.  Previously, unknown_bootoption() was
the last parse_args() subfunction to run, and it carefully put back
the '=' character.  Now the ignore_unknown_bootoption() is the last
one to run, and it wasn't doing the necessary repair, so the
envp params ended up with the embedded NUL and were no longer
seen as valid environment variables by init.

Tested-by: Woody Suwalski <terraluna977@gmail.com>
Acked-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>