Add flash option using st-flash

When running make flash I get this error:
```
  FLASH	awesomesauce.elf
make: *** [../rules.mk:159: awesomesauce.flash] Error 1
```

From what I understand, there's probably something related to openocd. However, I use st-flash:
`st-flash write $(PROJECT).bin 0x8000000`

It would be nice to have this option in the makefile.