global: Use CONFIG_XPL_BUILD instead of CONFIG_SPL_BUILD

Complete this rename for all directories outside arch/ board/ drivers/
and include/

Use the new symbol to refer to any 'SPL' build, including TPL and VPL

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/README b/README
index 54e7231..c27c386 100644
--- a/README
+++ b/README
@@ -1418,13 +1418,13 @@
 		This only takes effect if the memory commands are activated
 		globally (CONFIG_CMD_MEMORY).
 
-- CONFIG_SPL_BUILD
+- CONFIG_XPL_BUILD
 		Set when the currently running compilation is for an artifact
 		that will end up in one of the 'xPL' builds, i.e. SPL, TPL or
 		VPL. Code that needs phase-specific behaviour can check this,
 		or (where possible) use xpl_phase() instead.
 
-		Note that CONFIG_SPL_BUILD *is* always defined when either
+		Note that CONFIG_XPL_BUILD *is* always defined when either
 		of CONFIG_TPL_BUILD / CONFIG_VPL_BUILD is defined. This can be
 		counter-intuitive and should perhaps be changed.