For net boot setups it is useful to submit boot params like server or
bootfile over dhcp. To distinguish barebox from e.g. pxe machines,
a custom vendor id can be sent in dhcp discover/request messages.

E.g. the ISC dhcp server can be configured with

| if substring(option vendor-class-identifier,0,8) = "barebox:" {
|       next-server             192.168.3.24;
|       server-name             "192.168.3.24";
|       option tftp-server-name "192.168.3.24";
|       option root-path = concat("/srv/sysroots/by-mac/",
|          binary-to-ascii (16, 8, "-", substring (hardware, 1, 6)));
| }

to sent boot params which are valid for barebox hosts only.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Jean-Christophe PLAGNIOL-VILLARD:
 - update the use dhcp command option
 - support to set the vendor via env dhcp_vendor_id
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>