OpenSystemsDevelopment / usbboot

Initial BETA release for secure-boot.

secure-boot adds two new sub-directory to usbboot:

* secure-boot-recovery is used to create an EEPROM signed with
  the customer's RSA private key and configures the 2711 OTP
  settings permenantely require signed boot images.
  See secure-boot-recovery/README.md
* Once secure-boot is enabled the CM4 MSD mode firmware must
  also be signed with the customer's RSA private key.
  See secure-boot-msd/README.md

N.B The revoke_devkey and program_jtag_lock are NOT enabled in
this initial BETA release.

* Fix check against NULL for a static var actually looking for an empty c string.

* Update .gitignore for code change moving msd firmware to static vars.

* Fix support for OS X and *BSD after adding static vars for msd firmware.

This makes "git status" ignore some generated files that shouldn't be
added.