vsprintf.c: add EFI device path printing

For debugging efi_loader we need the capability to print EFI
device paths. With this patch we can write:

    debug("device path: %pD", dp);

A possible output would be

    device path: /MemoryMapped(0x0,0x3ff93a82,0x3ff93a82)

This enhancement is not available when building without EFI support
and neither in the SPL nor in the API example.

A test is provided. It can be executed in the sandbox with command
ut_print.

The development for EFI support in the sandbox is currently in
branch u-boot-dm/efi-working. The branch currently lacks
commit 6ea8b580 ("efi_loader: correct DeviceNodeToText
for media types"). Ater rebasing the aforementioned branch on
U-Boot v2018.01 the test is executed successfully.

Without EFI support in the sandbox the test is simply skipped.

Suggested-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>