runqemu: Add option for custom BIOS directory

runqemu: Add option for custom BIOS directory

Add support to specify a directory for custom BIOS, VGA BIOS and
keymaps as supported by qemu (-L option). Even though this can be
done through qemuparams, having this option provides better user
experience by not having to specify a long and cluttered path along
with other qemuparams that the user might want to specify.

This new options assumes first that the path provided is relative to
OECORE_NATIVE_SYSROOT and will check whether it exists before proceeding.
If not, it will treat the provided path as absolute. This provides
the user flexibility to use BIOS binaries generated inside or outside
the OE build environment.

(From OE-Core rev: d302f5683dd736ac4cd4b601a046d22000d41e68)

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>