net/bootp: add VCI support for BOOTP also

Vendor Class Identifier option is common to BOOTP and DHCP and
can be useful without PXE. So send VCI in both BOOTP and DHCP
requests if CONFIG_BOOTP_VCI_STRING is defined.

Signed-off-by: Ilya Yanok <ilya.yanok@cogentembedded.com>
Signed-off-by: Tom Rini <trini@ti.com>