OpenSystemsDevelopment / usbboot

Typo fixes from @dp111 (who obviously has too much time on his hands).

recovery.bin prevent revoke_devkey from being set before the
customer key has been programmed.

Enable the recovery config.txt options to revoke the ROM
development key which permentantely locks the device into
secure-boot mode.

WARNING: This also prevents the bootloader from being downgraded
to older releases which don't support secure-boot.

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.