Update MIMXRT clocking to hopefully fix deep sleep, fix boot issues on Teensy4 (#130) ...

* Fix incorrect image length in header preventing boot

* Fix ncache region being located in SEMC even on boards without it

* Don't reserve RAM for ncache anymore

* Fix crash when activating BufferedSerial on Teensy D0 and D1.  Also fix incorrect daisy register mapping.

* Fix missing space in linker script

* Start on fixing incorrect MIMXRT clock frequency

* LPM working!

* Clean up LPM and clocking

* Add teensy model identifier

* Fix license headers

Fix variable name conflict with toolchain file

Change toolchain file to load core and toolchain options *before* compilers are loaded.

CMake: add mbed-os and mbed-baremetal targets ...

mbed-os consists of mbed-core and mbed-rtos
mbed-baremetal consists of mbed-core

The main change is for mbed-core. Changing from object library to be interface. This way it allows us to do the above to have 2 main targets for users to use.

This should be backward compatible change as mbed-os target we used contains the same files/options as previously set.

CMake: Move modules under tools subdirectory