Update FLASH_SIZE_BYTES to avoid assert on > 2MB

The SDK would assert if a flash write/erase happened past the end of the
flash space it was built with.  So, 8MB chips would crash when accessing
offset 3MB, for example.

Avoid this by specifying the maximum flash available so the SDK won't
trip.

Fixes #76

pico-sdk-lib/CMakeLists.txt

@@ -15,6 +15,7 @@ add_library(pico STATIC)
 
 target_compile_definitions(pico PUBLIC
 	PICO_PRINTF_ALWAYS_INCLUDED=1
+	PICO_FLASH_SIZE_BYTES=16777216
 )
 
 target_link_libraries(pico