cmd: Fix license command

The license command isn't usually built and has a few problems:
- The rules to generate license.h haven't worked in a long time,
  re-write these based on the bmp_logo.h rules.
- 'tok' is unused and the license text size has increased
- bin2header.c wasn't grabbing unistd.h to know the prototype for
  read().

Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Tom Rini <trini@konsulko.com>