Add -Wdouble-promotion and -Wfloat-conversion for most ports to ban out
implicit floating point conversions, and add extra Travis builds using
MICROPY_FLOAT_IMPL_FLOAT to uncover warnings which weren't found
previously. For the unix port -Wsign-comparison is added as well but only
there since only clang supports this but gcc doesn't.
This is an extremely minimal port to the NXP i.MX RT, in the style of the
SAMD port It's largely based on the TinyUSB mimxrt implementation, using
the NXP SDK. It currently supports the Teensy 4.0 board with a REPL over
the USB-VCP interface.
This commit also adds the NXP SDK submodule (also from TinyUSB) to
lib/nxp_driver.
Note: if you already have the tinyusb submodule initialized recursively you
will need to run the following as the tinyusb sub-submodules have been
rearranged (upstream):
git submodule deinit lib/tinyusb
rm -rf .git/modules/lib/tinyusb
git submodule update --init lib/tinyusb
