vsprintf.c: correct printing of a NULL device path

When printing '%pD' with a value of NULL we want to output
'<NULL>'. But this requires copying to buf. Leave this
to string16.

A unit test is supplied which relies on EFI support in the sandbox.

The development for EFI support in the sandbox is currently in branch
u-boot-dm/efi-working. The branch lacks commit 6ea8b580 ("efi_loader:
correct DeviceNodeToText for media types"). Ater rebasing the aforementioned
branch on U-Boot v2018.01 and adding 256060e4 and this patch the test
is executed successfully.

Fixes: 256060e4 (vsprintf.c: add EFI device path printing)
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>