imports.mak should be excluded from git tracking

[nsaulnier-ti]

each person's imports.mak folder will be custom to their computer. However, git is currently tracking this file, leading to several issues:

1. The individual imports.mak should not get pushed back to the main branch, but we see imports.mak as one of the modified files in multiple pull requests (e.g., Am263x: PRU-I2S #8, Added AM26x support for Open PRU #9, Added Basic PRU Modules for AM26x #25)

2. If changes to imports.mak are NOT committed, then it blocks the ability to do critical git functions, like git pull --rebase and git rebase -i.

SOLUTION:
Provide a file with default settings, imports.mak.default. This file IS tracked in git, allowing us to update the template in the future.

Provide instructions to users to copy imports.mak.default to a new file called imports.mak, and make any computer-specific changes. That allows individual settings to coexist with the overall repository, as well as being backwards compatible with everyone's current repositories.