docker: Add common.rc

"requires" checks the "FEATURE" environment for specified prerequisits,
and skip the execution of test if not found.

"build_qemu" is the central routine to compile QEMU for tests to call.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1464755128-32490-8-git-send-email-famz@redhat.com
