[feat] Read from and write to the SD card

[hgruniaux]

The Raspberry PI has a SD card reader/writer. We should have an interface to it.

Moreover, the kernel is stored in this SD card. Being able to read from the SD card allows to extract other data from the SD (like the initial file system structure or the init program).

Tutorial:

• Reading: https://github.com/bztsrc/raspi3-tutorial/tree/master/0B_readsector
• Writing: https://github.com/bztsrc/raspi3-tutorial/tree/master/15_writesector

A filesystem driver should also be implemented like Ext2 (https://www.nongnu.org/ext2-doc/ext2.pdf).

According to https://github.com/bztsrc/raspi3-tutorial/tree/master/0E_initrd, it is also possible to load a TAR file from the SD card to memory (using the bootloader) to be used by the kernel. And a TAR reader is trivial to implement.