fs: expose generic ramdisk/flash disk access functions

This patch removes "FAT" from  files names, config names and
symbol names so as to avoid confusion, when other modules use
them in a more generic way.

Also flash_disk_access.c and ram_disk_access.c now exports generic
disk_access symbols as defined in include/disk_access.h rather
than FAT specific ones in fs/fat_diskio.h. Thus modules like
USB which need to use disk_access interface is not dependent
on symbols from ELMChan FAT module. Also fat_diskio.h
is removed.

Further the shim between ELM chan and Zephyr is modified as
per these changes.

Change-Id: Ifd80f14a629e467ee9c7a9aaff8a4896eed11982
Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>