scripts/rpm2cpio.sh: Use bzip2 instead of bunzip2

bzip2 is in HOSTTOOLS already and used in few other places already.
This fixes bin_package class for RPM packages without adding bunzip2 to
HOSTTOOLS.

Signed-off-by: Pavel Zhukov <pavel@zhukoff.net>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>