* Move filesystem mounting code into a separate file to simplify the
main source file.
* Clean up SDMMC and SDSPI related code. Move pin configuration into
Kconfig.
* Use same base_path ('/data') for either SPIFFS or SD. Remove the
check in file_server.c about the base path.
* Update README according to the changes above.
This change prevents buffer overflows in case of really long file paths.
Other changes:
* Remove query (?) and fragment (#) component from URI when converting to file path
* /index.html and favicon.ico can be overridden by files with same name and path in SPIFFS
* README.md updated
