[MIPS] Au1xx0: fix prom_getenv() to handle YAMON style environment

Alchemy boards use YAMON which passes the environment variables as the
tuples of strings (the name followed by the value) unlike PMON which
passes "name=<val>" strings.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>