Including external ini files for targets and arches

***In GitLab by @a3f on Apr 7, 2021, 11:29 GMT+1:***

I tested use of tuxmake with [barebox](barebox.org/) and found that including external ini files would
be very useful:

- barebox doesn't install to `arch/$ARCH/boot/`, but to `images/`. A custom external `target.ini` could account for this.
- Compiler toolchain prefixes. I use [OSELAS.Toolchain](https://www.pengutronix.de/en/software/toolchain.html) which e.g. uses a `CROSS_COMPILE=arm-v7a-linux-gnueabihf-` prefix. Including an external `arch.ini` would allow me to customize this.

Thoughts on adding this or is this perhaps already possible?

*Migrated from GitLab: https://gitlab.com/Linaro/tuxmake/-/issues/132*

Start by examining how target.ini and arch.ini settings are handled, including the target output path and CROSS_COMPILE values described in the issue. Define how external files would be supplied and verify that a custom target.ini can select images/ and a custom arch.ini can provide the OSELAS.Toolchain prefix.

Written by the indexing model from the issue text.