qemuriscv: check serial consoles w.r.t. /proc/consoles

qemuriscv enables hvc0 along with ttyS0, however its not enabled in
/proc/consoles, getty tries to enable it in inittab and erroring out

Fixes below message with sysvinit

INIT: Id "hvc0" respawning too fast: disabled for 5 minutes

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>