### Description
Traceback:
```
Merging Regions
Filling region bootloader with mbed-cloud-client-example/mbed-os/features/FEATURE_BOOTLOADER/targets/TARGET_Freescale/TARGET_MCUXpresso_MCUS/TARGET_MCU_K64F/TARGET_FRDM/mbed-bootloader-k64f-block_device-sotp-v3_4_0.bin
Padding region bootloader with 0x9a4 bytes
Traceback (most recent call last):
File "mbed-cloud-client-example/mbed-os/tools/make.py", line 293, in <module>
ignore=options.ignore
File "mbed-cloud-client-example/mbed-os/tools/build_api.py", line 548, in build_project
merge_region_list(region_list, res, notify)
File "mbed-cloud-client-example/mbed-os/tools/build_api.py", line 423, in merge_region_list
_fill_header(region_list, region).tofile(header_filename, format='hex')
File "mbed-cloud-client-example/mbed-os/tools/build_api.py", line 380, in _fill_header
header.puts(start, struct.pack(fmt, time()))
struct.error: required argument is not an integer
```
reason: `time()` returns a float. So the fix is to force it to be an `int`.
### Pull request type
[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Breaking change