framework-cmsis depends on framework-mbed

[tve]

The framework-cmsis comes with only a very small number of linker scripts. But it does try to match the board's variant with the linker scripts included in framework-mbed, see https://github.com/platformio/platform-ststm32/blob/develop/builder/frameworks/cmsis.py#L64
It took me a while to figure this out and I would have appreciated if either: framework-cmsis depends on framework-mbed and thus pio automatically installs the latter, or if the documentation of framework-cmsis prominently described this dependency.

[ivankravets]

@valeros could we make CMSIS independent?

[ivankravets]

Resolved in 5532cfe