iMXRT support

[thebolt]

This adds support for ARMv7M / iMXRT as well as a few minor bugfixes found while doing the port.

[codecov[bot]]

Codecov Report

Merging #28 (1785dd2) into master (ca877e2) will decrease coverage by 1.61%. The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
- Coverage   68.15%   66.54%   -1.61%     
==========================================
  Files          54       54              
  Lines        2795     2828      +33     
==========================================
- Hits         1905     1882      -23     
- Misses        890      946      +56     

Files	Coverage Δ
src/crypto.c	35.63% <100.00%> (-31.79%)	:arrow_down:
src/drivers/crypto/mbedtls/mbedtls_pb.c	94.18% <ø> (ø)
src/main.c	50.00% <ø> (ø)
src/boot/boot.c	75.17% <42.10%> (-0.39%)	:arrow_down:
src/boot/image_helpers.c	71.42% <22.22%> (-11.91%)	:arrow_down:

... and 1 file with indirect coverage changes