Introduced jtag_init and "jtag arp_init" to allow target scripts more control...

Introduced jtag_init and "jtag arp_init" to allow target scripts more control over how OpenOCD starts up and initializes the target.

git-svn-id: svn://svn.berlios.de/openocd/trunk@2805 b42882b7-edfa-0310-969c-e2dbd0fdcd60