Simple flake builds buildroot projects on nixos.
Buildroot hardcodes /usr/bin/ paths, which is why this project uses
a FHSuserEnv instead of a basic mkShell.
The cross toolchain used is AARCH64 but can be modified for other
architectures by modifying the pkgsCross path to the cross compiling gcc.
GarlicOS additionally requires openssl_1_1 but because it is deprecated in nixos I modified garlicos_armhf_defconfig and set BR2_PACKAGE_OPENSSL=y.