Added `node.egc.meminfo()` to expose LVM usage (to make the regular
`node.egc.ON_MEM_LIMIT` option usable).
Extended the `node.egc.ON_MEM_LIMIT` option to also take negative limits,
in which case that's taken as a request to keep a certain amount of heap
available for non-Lua use.
* Add FatFs
* enable BUILD_FATFS for all-module build
* push vfs into rest of firmware
* align maximum filename length
* increase timeout for acmd41 during card initialization
* switch from DOS to Unix path semantics chdrive() is substituted by chdir()
* update to fatfs R.012a incl. patches 1-6
* add callback for rtc provisioning in file
* update docs