Bluetooth: storage: Add basic support for internal storage

Add basic implementation of an internal storage handler that uses the
local file system. The root directory for all Bluetooth related data
is /bt. Each remote device has its own subdirectory and each key its
own file. This helps keep the implementation very simple, but does
come with the meta-data overhead for each file.

As an example, the value of a key 0x0001 for a device with a static
random address cc:11:22:33:44:55 would be stored in the following
file: /bt/cc11223344551/0001. Local values such as the identity
address are stored directly under /bt with a file name that matches
the key the same way as remote-device files.

For full functionality the implementation requires a file system that
can support file/directory names of up to 13 characters in length. If
the file system supports less than that (as is the case with FAT12)
then only local values can be stored (in /bt/abcd). Local values
include the identity address as well as the local IRK.

Jira: ZEP-181

Change-Id: I7dc696af6353a154cb00dcd01a5f4ac3d7127e6b
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>