bb-show: expand specified variable names
bb-show: expand specified variable names

This is useful so we can ask for vars like `FILES_${PN}` or
`QEMU_EXTRAOPTIONS_${PACKAGE_ARCH}`.

Signed-off-by: Christopher Larson <4c8c411260e601995081b113a5ef94a26eff2b25@mentor.com>

kergoth/bb,rossburton/bb,kergoth/bb,rossburton/bb