pico: add a BLIT_BOARD_x define for the selected board

To hopefully avoid more code relying on internal #defines to guess the board type...

(DISPLAY_ST7789 != PicoSystem)

32blit-pico/CMakeLists.txt

@@ -69,8 +69,10 @@ if(DEFINED PICO_ADDON)
     # for boards that don't have a board in the pico sdk
     # (usually because they are an add-on for a regular pico)
     set(CONFIG_PATH ${CMAKE_CURRENT_LIST_DIR}/board/${PICO_ADDON})
+    set(BOARD_ID ${PICO_ADDON})
 else()
     set(CONFIG_PATH ${CMAKE_CURRENT_LIST_DIR}/board/${PICO_BOARD})
+    set(BOARD_ID ${PICO_BOARD})
 endif()
 
 if(EXISTS ${CONFIG_PATH}/config.cmake)
@@ -89,6 +91,10 @@ if(EXISTS ${CONFIG_PATH}/config.h)
     list(APPEND BLIT_BOARD_DEFINITIONS "BLIT_BOARD_CONFIG=\"${CONFIG_PATH}/config.h\"")
 endif()
 
+# board id definition
+string(TOUPPER ${BOARD_ID} BOARD_ID)
+list(APPEND BLIT_BOARD_DEFINITIONS BLIT_BOARD_${BOARD_ID})
+
 # default drivers
 if(NOT BLIT_AUDIO_DRIVER)
     set(BLIT_AUDIO_DRIVER "none")