Fix stm32f3discovery flash programming
Fix stm32f3discovery flash programming

Fixes two issues:

1) Flash size was declared with the wrong size; since the bsp partitions
   where declared inside the valid flash size this fix is only for
   correctness.
2) Re-enable HSI clock. The flash programming interface on the F3 family
   is directly connected to the HSI clock generator, so without it
   enabled, trying to run any app that requires flash programming (eg
   slinky) would result in a hardfault.

Signed-off-by: Fabio Utzig <66676b1ceaf93296e098708c09494361103aa635@apache.org>

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core