Enforce E501: Limit the line length to 79 for files in pmb/parse - part 2

Made changes to limit the line length in following files for #1986 (closed),

• pmb/parse/bootimg.py
• pmb/parse/depends.py
• pmb/parse/kconfig.py
• test/test_parse_depends.py

Added the above files in E501 flake8 command list. Substitute f-string for string concatenation.