stlink: separate stlink core from USB functions
the introduced stlink_backend_s struct provides an API to separate USB
internals from stlink core.
this separation aims to ease:
- stlink-server integration [1]
- stlink driver split into modules:
- stlink_core
- stlink_usb
- stlink_tcp [1]
[1] refer to http://openocd.zylin.com/#/c/5633/
Change-Id: Iff6790942612ce1769ec4c75990914534e5e9e24
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5632
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>