[POWERPC] bootwrapper: serial_console_init() fixes

1. Search the entire compatible list for serial devices.

The serial code previously did a simple strcmp on the compatible
node; this fails when the match string is not the first compatible
listed.  Use dt_is_compatible() instead.

2. Don't call serial_edit_cmdline if getc isn't defined.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>