dpdkbuild: completely rebuild dpdk on SPDK config change

Configuring SPDK with different options might result
in enabling different CONFIG_RTE_LIB* options, but since
it does not modify any DPDK config files, DPDK doesn't
get rebuilt and keeps using older, potentially outdated
build options.

We'll now clear dpdk/build whenever SPDK config is modified,
which will result in automatically re-configuring DPDK.

Change-Id: I9c7c3434dc317e4f47767d7b3df3df48db2b0d5b
Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/460563


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>