guidance on implementing custom loaders

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a loader is just a program that

• parses config/binary formats
• loads data to memory
• uses kexec and reboot syscalls

u-root already has lots of parsers, and different OS kernels have different expectations from the loader, e.g. initial page table setup (FreeBSD, Haiku).

Clarify how instead of chaining a standalone binary a loader is implemented as a Linux userland program for LinuxBoot.

We may want to hint on loading other Linux programs from external sources instead of just kernels.