hp820g2: allow building, but don't do release ROMs

at present, the inject scripts compress refcode in a way
that is not reproducible, so there's no way to verify
that the firmware is correct, via checksum verification,
when injecting vendor code on release images

the lack of reproducibility in recompression will have to be
addressed, but the issue is that lbmk does not provide its own
sources for compression utilities, instead opting to use the
system's own compression utility

so the solution might be for lbmk not to use the host's utility,
and compile its own, or insert the refcode uncompressed. for now,
simply disable the hp 820 g2 target in libreboot releases

this uses the same logic recently implemented for excluding
mrc-based haswell images in libreboot releases

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/coreboot/hp820g2_12mb/target.cfg

@@ -5,3 +5,4 @@ payload_grub_withseabios="y"
 payload_seabios="y"
 payload_memtest="y"
 grub_scan_disk="ahci"
+release="n"