upgrade: Code cleanup and add logs for u-boot env update
This is the code cleanup continuation from !361 (merged):
- Line trim is no more needed during sysupgrade because the input file is already in valid form. The script iopsys-filter-uboot-env.sh is responsible to generate output file with proper line format.
- Add log messages regarding adding a new value and new variable.
- Add log message regarding finish of update u-boot env task.