pio exec: Run custom toolchain commands

[mamins1376]

Feature Request: Support pio exec command, which is useful to run binaries associated with the current project, without getting involved with including toolchain directories in $PATH for each project.

As an example, for stm8 platform, there are sdcc, stm8flash and stm8-binutils which is a powerful combination. Problem is, without adding their relevant prefixes in the path, one is going to have a painful experience. Either they can include them in the shell (possibly automating this by sourcing a script), or call them by their path. Both have their flaws: inclusion is prone to versioning mismatch and loss of cross platform compatibility, while the second one is tedious and repetitive.

My proposal is to add support for pio exec cmd --arg (sub)command, which is just a shortcut for PATH=~/.platformio/...:$PATH cmd --arg at it's core.

[ivankravets]

Thanks for filing this feature request. We have recently discussed it with @vshymanskyy and it would be great to have this functionality.

[ivankravets]

See docs https://docs.platformio.org/en/latest/core/userguide/pkg/cmd_exec.html

@mamins1376 , @vshymanskyy , please re-test with pio upgrade --dev.