vppinfra: add defines for bihash add and delete

vppinfra: add defines for bihash add and delete

When calling the bihash_add_del... functions some callers add a comment
beside the value to indicate this is the is_add param. Make the code
easier to read by adding defines for add and delete that the callers
can use instead of having to use 0 or 1.

Type: improvement
Signed-off-by: Paul Atkins <patkins@graphiant.com>
Change-Id: Iab5f7c8e8df12ac62fc7e726ca1798622dcdb42c