This change allows the time for which shell banners are displayed to
be configured in the config.h file.  The ability to access the shell
can also be effectively disabled by setting this timeout to zero.

Signed-off-by: Alexey Zaytsev <alexey.zaytsev@gmail.com>

list.

Added FEATURE() macros to most relevant (non-driver) files.

and generate DHCP options to indicate features to DHCP server (and to
PXE NBPs).

it's a serial console and remembers state from the previous program
run.