Introduce CONFIG_XPL

We have the term 'SPL', which has a dual meaning. It is both a particular phase of U-Boot (the one that loads U-Boot proper) and a generic name for any pre-proper phase.

You can see that in a few areas, but for example CONFIG_SPL_BUILD is enabled for TPL and VPL builds, not just SPL.

I propose to rename the generic term from SPL to xPL (meaning any PL phase), leaving SPL to just refer to the phase before U-Boot proper.

The symbol would be CONFIG_XPL but in documentation we would talk of xPL, with a lower-case X, so it is more obvious that it refers to any phase.

Discussion here: https://lore.kernel.org/all/CAFLszTiywgEjTB2s4Zmz4vcmqLhRXLcbTnJGyig3b38c9Y2KUw@mail.gmail.com/T/