Adding support for NUCLEO-F103RB

[Gregwar]

This adds support for the NUCLEO-F103RB boards, using changes introduced by @mbolivar allowing boards to override clocks (see #102)

Changes are:

• It adds .hex output in the build/ directory, that is compliant with openocd
• It adds comments in the Makefile to allow overriding of the STM32_PCLK* using CFLAGS
• It adds nucleo board support, overriding clocks to use the internal oscillator

Note that since there is no bootloader, you'll also have to run make jtag to build a valid image for the board.

[mbolivar]

I've merged this as two separate commits (one to the build system, and the other adding your board files). I didn't take the comment in the top-level Makefile though, as I don't want to encourage more use of STM32_PCLKx than we have to. Those were never a good idea...

[Gregwar]

Ok There is some kind of redundancy anyway:

• The actual clock settings
• The board CYCLES_PER_MICROSECONDS
• The PCLKx and delay constant

How do you suggest to deal with that?

[mbolivar]

Not sure. Ideally we shouldn't assume fixed clocks anywhere, though that will make it impossible to write a delay loop that works without systick (and thus in interrupt context). On Nov 27, 2014 1:10 AM, "Grégoire Passault" notifications@github.com wrote:

Ok There is some kind of redundancy anyway:

• The actual clock settings
• The board CYCLES_PER_MICROSECONDS
• The PCLKx and delay constant

How do you suggest to deal with that?

— Reply to this email directly or view it on GitHub https://github.com/leaflabs/libmaple/pull/103#issuecomment-64749912.