when overriding default tool names don't add cross-prefix

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: Stuart Yoder <stuart.yoder@freescale.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>