pico: add define for device id

We've got two incompatible devices here now

32blit-pico/CMakeLists.txt

diff --git a/32blit-pico/CMakeLists.txt b/32blit-pico/CMakeLists.txt
index 7f343a9..3c6ab0f 100644
--- a/32blit-pico/CMakeLists.txt
+++ b/32blit-pico/CMakeLists.txt
@@ -236,6 +236,7 @@
 
         target_compile_definitions(${NAME} PRIVATE ${BLIT_BOARD_DEFINITIONS}) # need these for framebuffer config
         target_compile_options(${NAME} PRIVATE -ffunction-sections -fdata-sections)
+        set_source_files_properties(${STARTUP_SRC} PROPERTIES COMPILE_DEFINITIONS DEVICE_ID=2)
 
         # minimal pico-sdk libs
         target_link_libraries(${NAME} boot_picobin_headers pico_bit_ops pico_clib_interface pico_cxx_options pico_divider pico_double pico_int64_ops pico_float pico_malloc pico_mem_ops pico_runtime_headers)

32blit-pico/startup.S

diff --git a/32blit-pico/startup.S b/32blit-pico/startup.S
index 8fe6d1a..26a637d 100644
--- a/32blit-pico/startup.S
+++ b/32blit-pico/startup.S
@@ -10,7 +10,7 @@
 .word _ZN4blit4tickEm // tick
 .word _entry_point // init
 .word __flash_binary_size
-.word 2 // device_id = 2 + padding
+.word DEVICE_ID // device_id + padding
 .hword BLIT_API_VERSION_MAJOR
 .hword BLIT_API_VERSION_MINOR
 .word __flash_binary_start // unused