Fixing the functionality of the "on" keyword in device configuration setup

[MadhavVP]

The config/DESCRIPTION file states that the "on" keyword is used to "allow a type name to have two implementations" on different hardware. However the parser does not actually support this functionality right now and instead just creates one implementation per type on whatever hardware is defined last in for that type in the config/Configuration file.

For example, if config/Configuration contains:

tty:
on uart
-i ttyinit -o ionull -c ionull
-r ttyread -g ttygetc -p ttyputc
-w ttywrite -s ioerr -n ttycontrol
-intr ttyhandler
on uart2
-i tttyinit2 -o ionull -c ionull
-r ttyread -g ttygetc -p ttyputc2
-w ttywrite -s ioerr -n ttycontrol
-intr ttyhandler

Only a device type of tty on uart2 will be made. Definition of a device of type tty on uart will return an error as an illegal device specification due to the device type tty on uart not existing.

This pull request fixes this issue by modifying the parser config/config.y to define a new device type every time the on keyword is used instead of just once every time a type name is used. All ith hardware implementations after the first are also given the functions of the i-1th hardware implementation as default in order to absolve the designer from having to retype any shared functions for every implementation definition.