Not rebuilding esp-idf and esp-idf-sys if using IDF_PATH, and changing esp-idf

[axos88]

workaround is to use cargo clean -p esp-idf-sys, but that will rebuild the entire esp-idf for every build.

[axos88]

Repro:

• check out esp-idf in a separate directory
• add ESP_IDF=
• cargo build
• change something in esp-idf
• cargo build

Expect to rebuild esp-idf-sys (and esp-idf), but that does not happen.

[ivmarkov]

This is a rare use case and I don't think there is an easy fix, so it will probably stay open for a while. #135 and #137 are more important.

[axos88]

No worries. At least it's now documented :)