mk: allow to override devel mode from environment

RTE_DEVEL_BUILD is set to := y in mk/rte.vars.mk, which makes it
impossible to override via an environment variable, and forces users
to pass it inline in the make call.
Use ?= instead to have it pick up the environment variable as well.

Signed-off-by: Luca Boccassi <lboccass@brocade.com>
Acked-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>