It is confusing to have both "$(PHASE_)" and "$(XPL_)" be used in our
Makefiles as part of the macros to determine when to do something in our
Makefiles based on what phase of the build we are in. For consistency,
bring this down to a single macro and use "$(PHASE_)" only.

Signed-off-by: Tom Rini <trini@konsulko.com>

Use XPL_ as the symbol to indicate an SPL build. This means that SPL_ is
no-longer set.

Signed-off-by: Simon Glass <sjg@chromium.org>

Add a fuzzer to test the vring handling code against unexpected
mutations from the virtio device.

After building the sandbox with CONFIG_FUZZ=y, the fuzzer can be invoked
with by:

   UBOOT_SB_FUZZ_TEST=fuzz_vring ./u-boot

This fuzzer finds unvalidated inputs in the vring driver that allow a
buggy or malicious device to make the driver chase wild pointers.

Signed-off-by: Andrew Scull <ascull@google.com>

Add the basic infrastructure for declaring fuzz tests and a command to
invoke them.

Signed-off-by: Andrew Scull <ascull@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>