make build system: Produce an error if BUILD_DIR_BASE is a Windows-style path
make build system: Produce an error if BUILD_DIR_BASE is a Windows-style path

Also fix an issue running menuconfig if BUILD_DIR_BASE is a relative path.

Closes https://github.com/espressif/esp-idf/issues/2736

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf