Implement an ELF Loader

[ziuziakowska]

This PR implements a simple ELF loader using the ELFIO header-library, exposed through a new bootstrap-elf command.

This loader first erases the sectors corresponding to each segment, and then programs them. For binaries which are quite sparse (the main use-case for this is OpenSBI+payload firmwares) this saves a lot of time by not writing gaps between segments.

With this ELF loader we should also be able to remove the dependency on objcopy'd .bin files in Mocha (1), (2).

Comparison (put the board into Bootstrap mode using ftditool gpio-write --ftdi Digilent 0 0, then 2 0 and then 2 1:

$ time build/ftditool bootstrap ~/mocha/build/sw/opensbi_with_uboot/fw_payload.bin --addr 0x80000000 --ftdi Digilent
Opening 200300BD8376A, channel 0
Loading [■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■] 100% 5.0 Mib in 17s, 2.4 Mib/s  

real    0m17.074s
user    0m0.371s
sys     0m4.224s

$ time build/ftditool bootstrap-elf ~/mocha/build/sw/opensbi_with_uboot/fw_payload.elf --ftdi Digilent
Opening 200300BD8376A, channel 0
Erasing [■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■] 100% 692.0 kib in , 21.1 Mib/s 
Loading [■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■] 100% 671.3 kib in 2s, 2.6 Mib/s 

real    0m2.736s
user    0m0.052s
sys     0m0.608s

[engdoreis]

Thanks for the nice feature!
I quick review, I'll give a more in depth review later.

[engdoreis]

is this copy due to the type difference?

[ziuziakowska]

Yes, the page program commands expect a std::span<uint8_t>, I think because the underlying transfer operations use a RW span and use an enum for the transfer direction type.