For net boot setups it is useful to submit boot params like server or
bootfile over dhcp. To distinguish diffrent type of OS running on the same hardware,
a custom vendor id can be sent in dhcp discover/request messages.

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

 | option client-uuid code 97 = { unsigned integer 8, string };
 | class "at91sam9x5ek" {
 |         match if substring (option vendor-class-identifier,0,20) = "barebox-at91sam9x5ek";
 |
 |         filename "/tftpboot/atmel/at91sam9x5/sam9x5ek/zImage";
 |         if substring (option dhcp-client-identifier,0,7) = "ser2net" {
 |                 filename "/tftpboot/atmel/at91sam9x5/sam9x5ek/zImage-ser2net";
 |         }
 |         if substring (option client-uuid,0,7) = "test" {
 |                 filename "/tftpboot/atmel/at91sam9x5/sam9x5ek/zImage-ser2net";
 |         }
 |         if substring (option user-class,0,4) = "toto" {
 |                 filename "/tftpboot/atmel/at91sam9x5/sam9x5ek/zImage-toto";
 |         }
 |         option tftp-server-name "192.168.200.98";
 |         option option-150 192.168.200.98;
 |         next-server 192.168.200.98;
 |         option root-path "192.168.200.98:/opt/work/buildroot/build/sam9x5/target";
 | }

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>