name the button correctly, allow to choose from ISP or PLAY button

[hg42]

It's a minor thing, but it cost me some time (I never used this mode, because it never worked and sdcard method did, but now I have to).
So I want to change this, in case someone else might get into the same trouble.

If I don't miss something the button to jump into dfu mode was changed without renaming the name in main.c.
This PR fixes the name to say what it does and allows to choose between ISP and PLAY button, which both works.

Note, I have an early Smoothieboard v1.0 (the one with A498x drivers) so I am not totally sure if it has a different pinout, but these pages at least show the same pins for those buttons:
http://smoothieware.org/smoothieboard-v1-old
http://smoothieware.org/pinout

[triffid]

Fwiw, ISP button can't be used for DFU because if ISP is held when coming out of reset, the chip's built-in serial bootloader is activated and this code can't run.

After startup, the pin can be used as ordinary GPIO, which is why we use it to trigger debug mode in smoothie proper as there's a button on it already.

It's theoretically possible to press the button after the on-chip bootloader checks the pin but before this code does, but that's a very tight window to hit.

[hg42]

Thanks for merging.

I only added the isp button as a way to document, it could theoretically be used (which was told to several people over the years) and that it is not used currently, but the play button. At least it was used at the beginning. Also it might not be obvious why the isp_btn_pressed was named as such before.