lib/vsprintf: allow printing upper case GUIDs

In the UEFI context GUIDs are expected to be rendered in upper case.

The patch uses the formerly unused bit 1 of the parameter str_format
of function uuid_bin_to_str() to indicate if we need upper or lower case
output.

Function uuid_string() in vsprint.c is adjusted to correctly set the bit
depending on the print format code.

%pUb: 01020304-0506-0708-090a-0b0c0d0e0f10
%pUB: 01020304-0506-0708-090A-0B0C0D0E0F10
%pUl: 04030201-0605-0807-090a-0b0c0d0e0f10
%pUL: 04030201-0605-0807-090A-0B0C0D0E0F10

Up to this point only a diagnostic message in mount_ubifs() using '%pUB' is
concerned by the change. Further patches are needed to adjust the UEFI
subsystem.

A unit test is provided inside the ut_print command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>