[uptime] new feature to track OT instance uptime (in msec) (#6968)

This commit adds a new module `Uptime` which tracks the number of
milliseconds since OpenThread stack initialization as an `uint64_t`
value. It also adds public OT APIs to get the current uptime value
(either as the number of milliseconds or in human-readable string
format like "2 days 12:45:12.762"). A CLI `uptime` command is also
added. This feature can be enabled using the newly added config
option `OPENTHREAD_CONFIG_UPTIME_ENABLE` (or the related CMake
`OT_UPTIME` option).

examples/README.md

@@ -65,3 +65,4 @@ This page lists the available common switches with description. Unless stated ot
 | TIME_SYNC | OT_TIME_SYNC | Enables the time synchronization service feature. **Note: Enabling this feature breaks conformance to the Thread Specification.** |  |
 | TREL | OT_TREL | Enables TREL radio link for Thread over Infrastructure feature. |
 | UDP_FORWARD | OT_UDP_FORWARD | Enables support for UDP forward. | Enable this switch on the Border Router device (running on the NCP design) with External Commissioning support to service Thread Commissioner packets on the NCP side. |
+| UPTIME | OT_UPTIME | Enables support for tracking OpenThread instance's uptime. |

examples/common-switches.mk

@@ -87,6 +87,7 @@ THREAD_VERSION            ?= 1.2
 TIME_SYNC                 ?= 0
 TREL                      ?= 0
 UDP_FORWARD               ?= 0
+UPTIME                    ?= 0
 RCP_RESTORATION_MAX_COUNT ?= 0
 
 
@@ -332,6 +333,10 @@ ifeq ($(UDP_FORWARD),1)
 COMMONCFLAGS                   += -DOPENTHREAD_CONFIG_UDP_FORWARD_ENABLE=1
 endif
 
+ifeq ($(UPTIME),1)
+COMMONCFLAGS                   += -DOPENTHREAD_CONFIG_UPTIME_ENABLE=1
+endif
+
 ifeq ($(DISABLE_BUILTIN_MBEDTLS),1)
 configure_OPTIONS              += --disable-builtin-mbedtls
 endif