When overriding a tool name via a shell variable, don't
tack on the cross-prefix.  This specifically allows the
pkg-config command to be overridden and work where it
does not exist in some cross build environments.
Signed-off-by: NStuart Yoder <stuart.yoder@freescale.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>