examples/ipsec-secgw: integrate inbound SAD

examples/ipsec-secgw: integrate inbound SAD

Integrate ipsec SAD support into secgw app:

1. Use SAD library for inbound SA lookup
2. Changes in struct sa_ctx:
  - sa array allocates dynamically depending on number of configured sa
  - All SA's are kept one by one without using SPI2IDX
3. SP's userdata now contain index of SA in sa_ctx instead of SPI
4. Get rid of SPI2IDX macro

Signed-off-by: Vladimir Medvedkin <vladimir.medvedkin@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>
Acked-by: Anoob Joseph <anoobj@marvell.com>