powerpc: Simplify and clean up the xmon terminal I/O

This factors out the common bits of arch/powerpc/xmon/start_*.c into
a new nonstdio.c, and removes some stuff that was supposed to make
xmon's I/O routines somewhat stdio-like but was never used.

It also makes the parsing of the xmon= command line option common,
so that ppc32 can now use xmon={off,on,early} also.

Signed-off-by: Paul Mackerras <paulus@samba.org>

include/asm-powerpc/xmon.h

@@ -7,7 +7,6 @@ struct pt_regs;
 extern int xmon(struct pt_regs *excp);
 extern void xmon_printf(const char *fmt, ...);
 extern void xmon_init(int);
-extern void xmon_map_scc(void);
 
 #endif
 #endif