net: bootp fix vci string on SPL-Boot

If CONFIG_CMD_DHCP is enabled, the vci (vendor-class-identifier) string
isn't inserted into the bootp-packet during SPL stage because the

CONFIG_BOOTP_VCI_STRING
instead
CONFIG_SPL_NET_VCI_STRING

We fix this with testing for CONFIG_SPL_BUILD and testing for existing
CONFIG_SPL_NET_VCI_STRING.

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Tom Rini <trini@konsulko.com>