Adds a help target to make.
Adds a help target to make.

help target searches and prints documentation strings specified with
### in the start.

Example:
### target: Help string for target

This will result in the following:
make help
target    Help string for target

Change-Id: Iad2f2c5a874b4a820f2246fb20cbe996742a7133
Signed-off-by: Piyush Itankar <4a4ff89c9da587217bb1deb9fbe4ebeb84f02484@google.com>

google/esh,google/esh