It is helpful to test that out-of-memory checks work correctly in code
that calls malloc().

Add a simple way to force failure after a given number of malloc() calls.

Fix a header guard to avoid a build error on sandbox_vpl.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Sean Anderson <seanga2@gmail.com>

Some files use U-Boot headers but still need to access the system
malloc(). Allow this by creating a new asm/malloc.h which can be used so
long as U-Boot's malloc.h has not been included.

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