sparc: Make '-p' boot option meaningful again.

If "-p" is given on the command line, clear the CON_BOOT
flag for the initial early boot PROM console.

This is necessary to try and see crash messages that occur
between the registry of the VT console and the probing of
the first framebuffer or serial console.  During this time
no console messages are emitted because the VT console
registry (even if no backend is registered to it) removes
the early console if CON_BOOT is set.

Signed-off-by: David S. Miller <davem@davemloft.net>