cmdline: support dynamic tokens

cmdline: support dynamic tokens

Considering tokens must be hard-coded in a list part of the instruction
structure, context-dependent tokens cannot be expressed.

This commit adds support for building dynamic token lists through a
user-provided function, which is called when the static token list is empty
(a single NULL entry).

Because no structures are modified (existing fields are reused), this
commit has no impact on the current ABI.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Olga Shern <olgas@mellanox.com>