[feature] Extended support for STM32H7 & rework of software reset

[Ant-ON]

This fix improves MCU detection (specifically H7). Patch will add a check of the current MCU instructions mode and switch to thumb mode. In some cases, MCU may wrongly be in ARM instruction mode.
Optimizes MCU firmware via gdb. Adds automatic detection of cache presence and breakpoints style.

I checked this changes on the debug boards that I have (F0/F1/F3/G4). Also this branch was checked for H7 in #1059.

(Closes #532, Closes #801, Closes #1059, Closes #1063)
(Partially fixes #868, Partially fixes #1008)

[Nightwalker-87]

@Ant-ON: Now, as our Travis CI is running again,
I found that this PR introduces some build failures when compiling with clang. Can you focus on that?
https://travis-ci.org/github/stlink-org/stlink/builds/747789017
Thanks in advance and sorry for the late feedback, we had no indication for that during review.

[Ant-ON]

@Nightwalker-87 I saw a link to the problem in a nearby discussion. I will try to fix it as soon as possible. Will I need to make a new pull request?

[Nightwalker-87]

Eh yes, as this one has been merged already, but you can continue on the used branch Ant-ON:try_h7_debug after pulling recent changes from our current develop branch to that personal feature branch. I'd propose to name the new PR "Extended support for STM32H7 (bugfix for clang)" and we shall be fine. I'll wait with any further contributions until you submitted a working patch. :-)