These patch provides the function of load a alternative initrd file

These patch provides the function of load a alternative initrd file
specified by the user via Yaboot prompt. To load the new file, just use
the "initrd=<name_of_file>" command. The <name_of_file> could include
all the directory path to the file.

Using only the "initrd" command, Yaboot will load the default kernel
specified in the configuration file and the initrd file specified by the
user. The user can, also, specify an initrd file to any other label of
the configuration file, just adding the "initrd" command before the
label's name.

Signed-off-by: Paulo Ricardo Paz Vital <vital@br.ibm.com>