esp32/esp32_common.cmake: Skip BTree module when requested.

This commit makes the BTree module truly optional, as it was
unconditionally enabled in the shared CMake script for the port.

This meant that if a board/variant did explicitly turn BTree off said
request was not honoured by the build system and the BTree module would
still be brought in.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

Author: agatti

ports/esp32/esp32_common.cmake

@@ -42,7 +42,9 @@ include(${MICROPY_DIR}/py/py.cmake)
 if(NOT CMAKE_BUILD_EARLY_EXPANSION)
     # Enable extmod components that will be configured by extmod.cmake.
     # A board may also have enabled additional components.
-    set(MICROPY_PY_BTREE ON)
+    if (NOT DEFINED MICROPY_PY_BTREE)
+        set(MICROPY_PY_BTREE ON)
+    endif()
 
     include(${MICROPY_DIR}/py/usermod.cmake)
     include(${MICROPY_DIR}/extmod/extmod.cmake)
@@ -276,7 +278,9 @@ target_include_directories(${MICROPY_TARGET} PUBLIC
 )
 
 # Add additional extmod and usermod components.
-target_link_libraries(${MICROPY_TARGET} micropy_extmod_btree)
+if (MICROPY_PY_BTREE)
+    target_link_libraries(${MICROPY_TARGET} micropy_extmod_btree)
+endif()
 target_link_libraries(${MICROPY_TARGET} usermod)
 
 # Extra linker options