Minor fixes and enhancements, including:

[psmears-cisco]

• Improve tool speed by performing some operations in parallel

• Add support for using "7zip" rather than "isoinfo" for extracting ISO contents (since the former supports images >2GB).

• When building inside a container, checksum the image produced before/after extracting it from the container, to detect corruption that can be caused if the disk is full

• Improve handling if no label exists on the ISO

• Improve the way the tool checks for its dependencies / sets up containers to have them, to ensure consistency.

• Clean up help strings

[cmohapat]

Have Reviewed from exr gisobuild point of view. Code fix is good w.r.t to exr gisobuild.