[build]: allow to specify timestamp format in the build log

[lguohan]

only simple/none are supported currently

Signed-off-by: Guohan Lu lguohan@gmail.com

- What I did

- How I did it

- How to verify it

lgh@gulv-vm3:/data2/sonic/stretch/sonic-buildimage$ BUILD_LOG_TIMESTAMP=simple make target/sonic-vs.img.gz
+++ --- Making target/sonic-vs.img.gz --- +++
EXTRA_JESSIE_TARGETS=sonic-vs.img.gz make -f Makefile.work jessie
make[1]: Entering directory '/data2/sonic/stretch/sonic-buildimage'
SONiC Build System

Build Configuration
"CONFIGURED_PLATFORM"             : "vs"
"CONFIGURED_ARCH"                 : "amd64"
"SONIC_CONFIG_PRINT_DEPENDENCIES" : ""
"SONIC_BUILD_JOBS"                : "1"
"SONIC_CONFIG_MAKE_JOBS"          : "16"
"SONIC_USE_DOCKER_BUILDKIT"       : ""
"USERNAME"                        : "admin"
"PASSWORD"                        : "YourPaSsWoRd"
"ENABLE_DHCP_GRAPH_SERVICE"       : ""
"SHUTDOWN_BGP_ON_START"           : ""
"ENABLE_PFCWD_ON_START"           : ""
"INSTALL_DEBUG_TOOLS"             : ""
"ROUTING_STACK"                   : "frr"
"FRR_USER_UID"                    : "300"
"FRR_USER_GID"                    : "300"
"ENABLE_SYNCD_RPC"                : ""
"ENABLE_ORGANIZATION_EXTENSIONS"  : "y"
"HTTP_PROXY"                      : ""
"HTTPS_PROXY"                     : ""
"ENABLE_SYSTEM_TELEMETRY"         : "y"
"ENABLE_RESTAPI"                  : "n"
"ENABLE_ZTP"                      : ""
"SONIC_DEBUGGING_ON"              : ""
"SONIC_PROFILING_ON"              : ""
"KERNEL_PROCURE_METHOD"           : "build"
"BUILD_TIMESTAMP"                 : "20200323.085529"
"BUILD_LOG_TIMESTAMP"             : "simple"

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)