Loader/NCCH: Add support for loading application updates (#2927)

* loader/ncch: split NCCH parsing into its own file

* loader/ncch: add support for loading update NCCHs from the SD card

* loader/ncch: fix formatting

* file_sys/ncch_container: Return a value for OpenFile

* loader/ncch: cleanup, always instantiate overlay_ncch to base_ncch

* file_sys/ncch_container: better encryption checks, allow non-app NCCHs to load properly and for the existence of NCCH structures to be checked

* file_sys/ncch_container: pass filepath as a const reference