stlink: separate stlink core from USB functions

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>