dpdkbuild: improve portability

Default to the more common Linux/GNU 'sed' edit in-place argument,
unless the platform is FreeBSD.

Tested by visually checking rte_build_config.h to confirm that the sed
in-place worked correctly.

Signed-off-by: Nick Connolly <nick.connolly@mayadata.io>
Change-Id: I68be69658930fb20318ac3aa2413bbf4a358e9bc
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/6531


Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Community-CI: Broadcom CI
Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>