net: bootp: Add environment variable for timeout period

There is currently one config option (CONFIG_NET_RETRY_COUNT) that
is available to tune the retries of the network stack.
Unfortunately, it is global to all protocols, and the value is
interpreted differently in all of them.

Add a new environment variable that directly sets the retry period for
BOOTP timeouts. If this new value is not set, the period is still derived
from the default number of retries, or from CONFIG_NET_RETRY_COUNT if
defined. When both the new variable is set and CONFIG_NET_RETRY_COUNT
is defined, the variable has precedence.

Signed-off-by: Alexandre Messier <amessier@tycoint.com>