[dhcp] Allow for variable encapsulation of architecture-specific options

[dhcp] Allow for variable encapsulation of architecture-specific options

DHCPv4 and DHCPv6 share some values in common for the architecture-
specific options (such as the client system architecture type), but
use different encapsulations: DHCPv4 has a single byte for the option
length while DHCPv6 has a 16-bit field for the option length.

Move the containing DHCP_OPTION() and related wrappers from the
individual dhcp_arch.h files to dhcp.c, thus allowing for the
architecture-specific values to be reused in dhcpv6.c.

Signed-off-by: Michael Brown <mcb30@ipxe.org>