net: dhcp: Split dhcp funcionality & add dhcp command ...

dhcp/bootp was bound to the command functionality.
This patch splits the dhcp command.
We are now able to use bootp without a shell.

This patch adds also a check for environment variables and globalvar.
So only when ENVIRONMENT_VARIABLES and GLOBALVAR is set, all received
dhcp data will be stored.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>