console: provide assured printing

Provide a new option to Mini-OS to compile with an assured
print implementation (CONFIG_CONSFRONT_SYNC). The default
implementation drops printing characters whenever the console
ring is full. When CONFIG_CONSFRONT_SYNC is enabled (disabled
by default), print will busy-loop and wait for free space on the
ring in order to print every character. This is useful for debugging
because it is ensured that all messages are print to the console
(e.g., stack traces on crashes). However, note that busy-loop might
slow down your application when it calls print too often or with
too big strings.

Fixes #6.