Prior to this fix the following would fail:
$ make build-TEENSY40/flexram_config.s
because it didn't create the build directory before generating the file.
Also, make `hal/resethandler_MIMXRT10xx.S` have an explicit dependency on
`flexram_config.s` rather than the latter just being forced to be built
before everything else.
Signed-off-by: Damien George <damien@micropython.org>