Add option to configure char sequence for stopping boot process

Sequence is stored in 'bootstopkey' environment variable, default
sequence is <ESC> key (\x1B).
Non-printable chars in env variable should be escaped
using (\x1F) notation. Max len of sequence 40 chars for env
variable, 20 chars for raw (unescaped) string.
Add new UART driver, support for configurable baudrates
Add (c) notices, changelog, version system