Encapsulate the jtag_trst and jtag_srst variables:

- Add accessor functions to return their value.
- Use new SRST accessor in cortex_m3.c and mips_m4k.c


git-svn-id: svn://svn.berlios.de/openocd/trunk@2157 b42882b7-edfa-0310-969c-e2dbd0fdcd60