all: Remove query-variants make target.

This is difficult to implement on cmake-based ports, and having the list
of variants in mpconfigboard.{cmake,mk} duplicates information that's
already in board.json.

This removes the existing query-variants make target from stm32 & rp2
and the definition of BOARD_VARIANTS from the various board files.

Also renames the cmake variable to MICROPY_BOARD_VARIANT to match other
variables such as MICROPY_BOARD. The make variable stays as
BOARD_VARIANT.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

extmod/extmod.cmake

@@ -62,7 +62,7 @@ if(MICROPY_PY_BTREE)
     set(MICROPY_LIB_BERKELEY_DIR "${MICROPY_DIR}/lib/berkeley-db-1.xx")
     string(CONCAT GIT_SUBMODULES "${GIT_SUBMODULES} " lib/berkeley-db-1.xx)
 
-    if(ECHO_SUBMODULES OR ECHO_QUERY_VARIANTS)
+    if(ECHO_SUBMODULES)
         # No-op, we're just doing submodule/variant discovery.
         # Cannot run the add_library/target_include_directories rules (even though
         # the build won't run) because IDF will attempt verify the files exist.