Move MakeFatImage out of esp.cpp

[Databean]

Bug: b/492644615

[3405691582]

I have some thoughts but since the change is primarily code motion we should revisit them later.

[Databean]

No rush on this change, what are your thoughts?

[3405691582]

Specifically: one (easier), we should label the flags to NewfsMsdos and MkfsFat so they'll be shared and read together, and two (harder), we should just not shell out to these commands and see if there's a library that does this for us.

Again: neither necessary for this change if you don't want to, since this is just code motion.

[Databean]

From a quick look Elm FatFs seems like a promising library and has already been imported internally as well. I think we can also use it to replace the invocations to mmd and mcopy which are used to place files into the filesystem.

The relevant part for integration is that it expects users to implement a few functions declared in the FatFs headers, e.g. dread:

DRESULT disk_read (
  BYTE pdrv,     /* [IN] Physical drive number */
  BYTE* buff,    /* [OUT] Pointer to the read data buffer */
  LBA_t sector,  /* [IN] Start sector number */
  UINT count     /* [IN] Number of sectros to read */
);

My preference would be an integration point around function pointers and opaque context objects rather than this BYTE pdrv to distinguish between different instances, but it's workable.

I'll file an issue to track this conversion: b/493268636