Modernize

[arkadijs]

1. Allocate user IRQ to avoid conflict / hard fault when other parts are changed and/or SDK features included.
2. Use *.cmake from SDK 1.5.1.
3. Allow for in-tree board definitions.
4. A distinctive CMake project name for IDE.
5. Include host/usbh.h instead of tusb.h where the former could be ifdef-ed out depending on the config. I believe this is a proper semantic / intention.
6. Overclocking now preserves correct peripherals clock (SPI).
7. Invoke irq_set_pending() every core1 loop. While invoking once is sufficient when all is going well, I see better stability when things go wrong. It's a cheap insurance that has no bad side effects.
8. I've experimented with sleep_ms() between vreg_set_voltage() and set_sys_clock_khz() and it did not resolve cold-start issue. TODO removed.