efi_selftest: support printing leading zeroes

Allow specifying the precision when printing integers, e.g.

efi_st_printf("%.4u-%.2u-%.2u\n", year, month, day);

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>