libpayload: time: split time.h from libpayload.h

Move time functions and prototypes from libpayload.h to time.h.
In a similar manner to other c libary headers, this change makes
porting existing applications to libpayload easier.

Change-Id: I71e27c6dddde6e77e0e9b4d7be7cd5298e03a648
Signed-off-by: Stef van Os <stef.van.os@prodrive-technologies.com>
Reviewed-on: https://review.coreboot.org/14437
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
2 files changed