target/target.c, jim_target_md refractored

target/target.c, jim_target_md refractored

 - Added a few lines of comment before the function explaining the usage and the
output generated by the command
 - Added a few lines of comment in the body explaining what is happening to improve
code readability
 - Renamed a few variables to improve readability:
    * a -> addr
    * b -> dwidth
    * c -> count
 - Added a new variable, named byte to contain the number of bytes to read, instead
of overwriting the count parameter, to avoid confusion between the two values.

Change-Id: I5828ec0f5aadaa39becec7b84f198756bb2c3d41
Signed-off-by: Vandra Akos <axos88@gmail.com>
Reviewed-on: http://openocd.zylin.com/665
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>