Makefile: Update $(RELEASENAME) to `flashrom-stable-xxx`

Change-Id: I4019f033787497e648f7e844677e2d0ab085323a
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/71800
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
1 file changed