[mellanox]: Enable fast boot mode for SX kernel (#853)

Author: oleksandrivantsiv

files/build_templates/swss.service.j2

@@ -18,6 +18,7 @@ ExecStartPre=/usr/bin/docker exec database redis-cli -n 1 FLUSHDB
 ExecStartPre=/usr/bin/docker exec database redis-cli -n 2 FLUSHDB
 
 {% if sonic_asic_platform == 'mellanox' %}
+Environment=FAST_BOOT=1
 ExecStartPre=/etc/init.d/sxdkernel start
 ExecStartPre=/usr/bin/mst start
 ExecStartPre=/sbin/modprobe i2c-dev