runqemu: Define OECORE_MACHINE_SYSROOT on setup_sysroot

runqemu: Define OECORE_MACHINE_SYSROOT on setup_sysroot

At least the OVFM (UEFI Firmware for Qemu and KVM) recipe stores the BIOS
under $OE_TMPDIR/sysroots/$MACHINE, now defined as OECORE_MACHINE_SYSROOT.
The latter is used when searching BIOS, VGA BIOS and keymaps. As a example,
to boot a OVFM BIOS, one can run the following command:

$ runqemu qemux86-64 core-image-minimal \
    biosdir=usr/share/ovmf  \
    biosfilename=bios.bin \
    nographic

Note the bios* parameters: these two are needed to specify the subfolder
(parent folder is OECORE_MACHINE_SYSROOT) and BIOS filename (without it,
it picks a BIOS named bios-256k.bin).

[YOCTO #5654]

Signed-off-by: Leonardo Sandoval <leonardo.sandoval.gonzalez@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>