"Basic" ST-Link/v2 support

[wldhx]

Implements ST-Link/v2, an ubiquitous and cheap (ca. $5 on Aliexpress) programmer, support for posix platforms.

• [x] "basic" .stlinkv2 targets: flash, reset, erase
• [x] debugging with gdb
• [x] "unitargets": Have stdtargets (like flash) default to stlink if PROGRAMMER = stlinkv2 is set
• [x] Add a README section

Also see #19.

[wldhx]

@brghena, @bradjc I think I'm more or less done - could you review?