spl: Add C-based runtime detection of SPL

The spl_phase() function indicates whether U-Boot is in SPL and before
or after relocation. But sometimes it is useful to check for SPL with
zero code-size impact. Since spl_phase() checks the global_data flags,
it does add a few bytes.

Add a new spl_in_proper() function to check if U-Boot proper is
running, regardless of the relocation status.

Signed-off-by: Simon Glass <sjg@chromium.org>