util/qemu: Add qemu make target

Add some mechanics to automatically have a `qemu` make target for
supported configurations. So with a QEMU target selected in Kconfig,
one would ideally only have to run `make qemu` to test things.

There are some notable variables that can be set or adapted in
`Makefile.inc` files, the make command line or the environment.

Primarily for `Makefile.inc` use:
 QEMU-y           the QEMU executable
 QEMU_CFG-y       a QEMU config that sets the available default devices,
                  used to run more comprehensive tests by default,
                  e.g. many more PCI devices

For general use:
 QEMU_ARGS        additional command line arguments (default: -serial stdio)
 QEMU_EXTRA_CFGS  additional config files that can add devices

 QEMU_CFG_ARGS    gathers config file related arguments,
                  can be used to override a default config (QEMU_CFG-y)

Examples:

  $ # Run coreboot's default config with additional command line args
  $ make qemu QEMU_ARGS="-cdrom site-local/grml64-small_2018.12.iso"

  $ # Force QEMU's built-in config
  $ make qemu QEMU_CFG_ARGS=

Change-Id: I658f86e05df416ae09be6d432f9a80f7f71f9f75
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46767
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

Makefile.inc

@@ -83,7 +83,7 @@ subdirs-y += src/superio
 subdirs-y += $(wildcard src/drivers/*) $(wildcard src/drivers/*/*) $(wildcard src/drivers/*/*/*)
 subdirs-y += src/cpu src/vendorcode
 subdirs-y += util/cbfstool util/sconfig util/nvramtool util/pgtblgen util/amdfwtool
-subdirs-y += util/futility util/marvell util/bincfg util/supermicro
+subdirs-y += util/futility util/marvell util/bincfg util/supermicro util/qemu
 subdirs-y += $(wildcard src/arch/*)
 subdirs-y += src/mainboard/$(MAINBOARDDIR)
 subdirs-y += src/security