doc: mxc_hab: Update i.MX HAB documentation

The README.mxc_hab is outdated and need improvements, add the following
modifications:

- Reorganize document and remove duplicate content
- Add CST download link
- Update CST package name
- Align command lines with CST v2.3.3
- Update U-Boot binary name
- Remove CSF padding since is not documented in AN4581

Signed-off-by: Breno Lima <breno.lima@nxp.com>