lwip: undiif: Fix debug options to unique *_DEBUG macros

Don't reuse *_DEBUG macros intended for other source files.

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>
lwip: Allow LWIP_PLATFORM_* output to use dprintf

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>
lwip/undiif: split arphdr._hwlen_protolen

lwIP already split this in etharp.h

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>
core/lwip/undiif: dprint MAC/ARP/IP/ICMP/TCP/UDP headers; poll debug

Encapsulate in #ifdef; use snprintf and 1 dprintf-like statement.
Print debug message that polling has been forced

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>
Allow for UNDIIF_ID_DEBUG in 1 mk line

Add comments and 1 line to mk/devel.mk to debug headers of all
packets sent/recieved through undiif.c.

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>