Fixed up repo:
- Add support for ECDSA P-256 with SHA256 from https://github.com/osm0sis/mkbootimg/commit/ea4ce75a86e42e5c3a9ec5dd94d4ea808d29973b

- Add support for SHA-256 hash algorithm from https://github.com/osm0sis/mkbootimg/commit/8f9a3db0fd0519f21bd567adceaa56f323a82e8a

- Modified mkbootimg.c to form unmkbootimg.c with dt support and fixed up minor errors to read the cmdline from both the image and the bootimg header.

- Partial changes to the makefile.

- And much much more to be wanting to list here but feel free to review this commit.