ppc/pegasos2: Fix spurious warning with -bios

ppc/pegasos2: Fix spurious warning with -bios

The -append option is currently not compatible with -bios (as we don't
yet emulate nvram so we can only put it in the environment with VOF).
Therefore a warning is printed if -append is used with -bios but
because the default value of kernel_cmdline seems to be an empty
string instead of NULL this warning was printed even without -append
when -bios is used. Only print warning if -append is given.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-Id: <483ac599a1407b766179aaea2794aed60cc09f53.1626367844.git.balaton@eik.bme.hu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>