Merge patch series "Switch to using $(PHASE_) in Makefiles"

Merge patch series "Switch to using $(PHASE_) in Makefiles"

Tom Rini <trini@konsulko.com> says:

This series switches to always using $(PHASE_) in Makefiles when
building rather than $(PHASE_) or $(XPL_). It also starts on documenting
this part of the build, but as a follow-up we need to rename
doc/develop/spl.rst and expand on explaining things a bit.

Link: https://lore.kernel.org/r/20250401225851.1125678-1-trini@konsulko.com