[build] allow specify build date time (#6856)

This commit adds a macro for specifying OpenThread build date time,
which can be empty so that this field can be omitted.

script/check-android-build

@@ -45,8 +45,26 @@ check_targets()
     done
 }
 
+check_datetime()
+{
+    local datetime
+
+    datetime="$(date)"
+    cat >openthread-config-datetime.h <<EOF
+#include <openthread-config-android.h>
+#define OPENTHREAD_BUILD_DATETIME "$datetime"
+EOF
+
+    OPENTHREAD_PROJECT_CFLAGS="-I$PWD -DOPENTHREAD_CONFIG_FILE=\\\"openthread-config-datetime.h\\\" \
+        -DOPENTHREAD_PROJECT_CORE_CONFIG_FILE=\\\"openthread-core-posix-config.h\\\"" \
+        make showcommands ot-cli
+    grep "$datetime" -ao "out/target/product/generic/system/bin/ot-cli"
+    make clean-ot-cli
+}
+
 main()
 {
+    OPENTHREAD_ENABLE_ANDROID_MK=1 ANDROID_NDK=1 check_datetime
     OPENTHREAD_ENABLE_ANDROID_MK=1 ANDROID_NDK=1 USE_OTBR_DAEMON=1 check_targets ot-cli ot-ctl
     OPENTHREAD_ENABLE_ANDROID_MK=1 ANDROID_NDK=1 check_targets ot-cli spi-hdlc-adapter
 }