readme: document NFMT and NCMD registers

This fully documents the NCMD register, and adds some more information
about what's in the NFMT register.
nand: modularize nand log dumper

We replace nand_log_dump with nand_log_foreach that calls a passed
function for each entry in the NAND log.  This allows us to reuse the
complicated log dumper code for multiple purposes, such as summarizing
the contents or printing the full contents.
debugger: add command to figure out NCMD register

This command will test every possible, legal value of the NCMD register.