lpc1768.cfg abstracted and moved to lpc17xx.cfg

- Moved variant-independent code to lpc17xx.cfg, which will be included from
lpc17??.cfg files automatically.
- lpc1768.cfg filled with variant-dependent code.

Change-Id: I7dabe6ed7da7be640ed38c13aaaa096b8796d9a0
Signed-off-by: Vandra Akos <axos88@gmail.com>
Reviewed-on: http://openocd.zylin.com/675


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>