Add Vignesh's check for existing include file

Ignore header files that are not in the kernel's top-level
directory because the local component header files will
be analysed in the component directory.

This modification was necessary to prevent git log failures
in later analysis.

Signed-off-by: Dean Jenkins <Dean_Jenkins@mentor.com>
list_includes.pl: did some tidy-ups

Improved the script to check that the script was being run
where the kernel include directory could be found. OK, it is
a simple check that the prefix directory can be found.

Reworked Vignesh's change to simplify the code.

Signed-off-by: Dean Jenkins <Dean_Jenkins@mentor.com>