powerpc: load arch specific scripts from common area

When assembling the final sysroot copy executable scripts and script
includes from a common architecture specific directory.

rootconf/<arch>/sysroot-lib-onie ---> $(SYSROOT)/lib/onie
rootconf/<arch>/sysroot-bin      ---> $(SYSROOT)/bin

The files found in the above directories are copied into the final
sysroot image.

This is the PowerPC implementation of this change.