stlink: simplify maintenance of version and features

stlink: simplify maintenance of version and features

The number of stlink firmware version is growing, each carrying new
features. Today's code has several check distributed here and there
and it's already hard to track them and verify the correctness.
The introduction of STLINK-V3 will make the situation much worst,
and the code much less readable.

Add a "flags" bitmask in the struct stlink_usb_version to allow
setting individual bits for each feature available or for specific
quirks and workarounds.
This patch does not implement setting nor testing "flags"; it would
be introduced in following patches, one bit at a time.

Change-Id: I09d78202646a6c8330731f8aa96dc9d295fa5655
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4706
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>