This is an open source implementation of 3DS loader system module--with
additional features. The current aim of the project is to provide a nice
entry point for patching 3DS modules.
Right now, this can serve as an open-source replacement for the built in loader.
There is additional support for patching any executable after it's loaded but
before it starts. For example, you can patch menu to skip region checks and
have region free game launching directly from the home menu. There is also
support for SDMC reading (not found in original loader implementation) which
means that patches can be loaded from the SD card. Ultimately, there would be
a patch system that supports easy loading of patches from the SD card.
You need a working 3DS build environment with a fairly recent copy of devkitARM, ctrulib, and makerom. If you see any errors in the build process, it's likely that you're using an older version.
Currently, there is no support for FIRM building, so you need to do some steps manually. First, you have to add padding to make sure the NCCH is of the right size to drop in as a replacement. A hacky way is this patch which adds junk data. Play around with the size value to get the NCCH to be the exact same size as the one found in your decrypted FIRM dump.
Once you have a NCCH of the right size, just replace it in your decrypted FIRM and find a way to launch it (for example with ReiNAND).