bootm: Add subcommands

Add the ability to break the steps of the bootm command into several
subcommands: start, loados, ramdisk, fdt, bdt, cmdline, prep, go.

This allows us to do things like manipulate device trees before
they are passed to a booting kernel or setup memory for a secondary
core in multicore situations.

Not all OS types support all subcommands (currently only start, loados,
ramdisk, fdt, and go are supported).

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

lib_ppc/bootm.c

@@ -117,6 +117,9 @@ int do_bootm_linux(int flag, int argc, char *argv[], bootm_headers_t *images)
 	char	*of_flat_tree = images->ft_addr;
 #endif
 
+	if ((flag != 0) && (flag != BOOTM_STATE_OS_GO))
+		return 1;
+
 	kernel = (void (*)(bd_t *, ulong, ulong, ulong,
 			   ulong, ulong, ulong))images->ep;