SW: Update greybus mikrobus and FW build workflow

Hi @jadonk ,

Can you review this PR, the main changes are updating the greybus-for-zephyr-mikrobus submodule, the previous version was not the latest one which caused the mikrobus sample to fail before. Once https://github.com/jadonk/greybus-for-zephyr/pull/3 is merged, greybus-for-zephyr-mikrobus submodule can be updated back to https://github.com/jadonk/greybus-for-zephyr

• updated the FW build workflow to upload only the necessary binaries/debug info(DTS, Kconfig) instead of uploading the entire build/ directory and also to make a FW release with these binaries for a tag.
• FW Release for testing: https://github.com/vaishnav98/beagleconnect/releases/tag/bcf-beta0.0.3
• minor update to cc2538-bsl.py so that uploading .bin file directly instead of being present in a folder, so that the below usage also works:

vaishnav@spectre:~/Downloads/bcf-firmware-release$ ./cc2538-bsl.py wpanusb_beagleconnect.bin 
Opening port /dev/ttyACM0, baud 50000
Reading data from wpanusb_beagleconnect.bin
Firmware file: Raw Binary
  ...
Verified (match: 0xc9f1e101)