powerpc-utils: optionally use librtas

Now that librtas is available, optionally use it with powerpc-utils to
enable it's RTAS dependant functionality.  It is used by default, as
most distros do, but made optional because it adds several new programs
and significantly increases the size of the installed package.

[Thomas: propagate the glibc dependency from the librtas package, as
suggested by Romain Naour.]

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>