This fixes possible issues when developers specify arbitrary partition files
using relative or absolute paths.
Additionally, hyphens in filenames are replaced with underscores to
avoid compilation warnings "ISO C++11 requires whitespace after the macro name"
Resolvesplatformio/platform-espressif32#787
This PR adds support for uploading additional flash images (e.g. Adafruit Tiny UF2 bootloader) specified in board manifests.
Additionally, the PR switches the PlatformIO CI script to the upstream version of the ESP32 dev-platform (basically reverts changes introduced in #5387 as they are no longer required).
