build: remove duplicated code

Hi :)

I noticed that Makefile contains duplicated code in some places and in this PR I tired to move this code to variable where possible.

Here is exactly what I did:

• jslint and jslint-ci had (and probably should have) the same list of directories to lint. This list was copy/pasted from one target to another. Just moved it to separated variable.

• targets that generates json and html docs. There was pretty big bash script with very small difference for these targets. Again, the command was just moved to separated variable.

There is no issue filled for this, but I hope the change can be useful. Thank you.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

build