README: Correct docs for CONFIG_SPL_BUILD

This option is defined in both SPL and TPL builds, so correct the docs
related to this. Also point to spl_phase() which is normally a better
option. Mention VPL as well.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>