core/lib/settings: Generalized Settings Manager to work on any platform

This commit moves the Settings Manager from the AVR codebase
into the Contiki core library. Any platform that implements
the Contiki EEPROM API can now use the Settings Manager's
key-value store for storing their persistent configuration info.

The Settings Manager is a EEPROM-based key-value store. Keys
are 16-bit integers and values may be up to 16,383 bytes long.
It is intended to be used to store configuration-related information,
like network settings, radio channels, etc.

 * Robust data format which requires no initialization.
 * Supports multiple values with the same key.
 * Data can be appended without erasing EEPROM.
 * Max size of settings data can be easily increased in the future,
   as long as it doesn't overlap with application data.

The format was inspired by the [OLPC manufacturing data format][].

Since the beginning of EEPROM often contains application-specific
information, the best place to store settings is at the end of EEPROM
(the "top"). Because we are starting at the end of EEPROM, it makes
sense to grow the list of key-value pairs downward, toward the start of
EEPROM.

Each key-value pair is stored in memory in the following format:

Order    | Size     | Name         | Description
--------:|---------:|--------------|-------------------------------
       0 |        2 | `key`        | 16-bit key
      -2 |        1 | `size_check` | One's-complement of next byte
      -3 |   1 or 2 | `size`       | The size of `value`, in bytes
-4 or -5 | variable | `value`      | Value associated with `key`

The end of the key-value pairs is denoted by the first invalid entry.
An invalid entry has any of the following attributes:

 * The `size_check` byte doesn't match the one's compliment of the
   `size` byte (or `size_low` byte).
 * The key has a value of 0x0000.

[OLPC manufacturing data format]: http://wiki.laptop.org/go/Manufacturing_data
core/lib/settings: Added `SETTINGS_CONF_SKIP_CONVENIENCE_FUNCS`.

Unfortunately, some platforms don't properly drop unreferenced functions,
so on these broken platforms we can save a significant amount
of space by skipping the definition of the convenience functions.
cpu/native: Add file-backed simulated EEPROM to native cpu.

This patch removes a defunct EEPROM implementation from the native
platform and provides a new EEPROM implementation for the native cpu.
The previous implementation appears to be vestigal.

This is useful for testing code which uses the EEPROM without running
the code on the actual hardware.

By default the code will create a new temporary file as the EEPROM
backing, reinitializing each time. If you would like to preserve the
EEPROM contents or specify a specific EEPROM file to use, you can set the
`CONTIKI_EEPROM` environment variable to the name of the EEPROM file you
wish to use instead. If it already exists, its contents will be used.
If it does not already exist, it will be created and initialized by
filling it with `0xFF`---just like a real EEPROM.

A new example is also included, which was used to verify the correctness
of the implementation. It can easily be used to verify the EEPROM
implementations of other targets.
Merge pull request #130 from darconeous/pull-requests/cpu-native-eeprom

cpu/native: Add file-backed simulated EEPROM to native cpu.
Merge pull request #144 from darconeous/pull-requests/settings-for-all-targets

core/lib/settings: Generalized settings manager to work on any platform