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.
Fix #183 (short term fix)

This is a temporary fix for #183, so that things can
build cleanly until the issue is fixed properly.

If RIMESTATS_CONF_ENABLED is 0, rimestats.foo will always
read as 0, since RIMESTATS_ADD(foo) doesn't do anything
Provide a macro to read rimestats when enabled
Rename RIMESTATS_CONF_ON to RIMESTATS_CONF_ENABLED
Replace reads of rimestats with RIMESTATS_GET(x)
Update to markdown

- Spellchecking
- Update plain text to github markdown
Rename to md
Update README.md

Minor markdown => md renaming
New Platform: TI CC2538 Development Kit

This commit adds cpu, platform and example files,
providing support for running Contiki on TI's cc2538 DK
Update of doxygen.to 1.8.1.2 using doxygen -u
Merge pull request #194 from g-oikonomou/cc2538-forward

New Platform: TI CC2538 Development Kit
Improve handling of CDC ACM line state

- For the CC2538, simplify handling of USB_CDC_ACM_LINE_STATE
  events. Ignore the Carrier Control (RTS) bit when receiving
  a SET_CONTROL_LINE _STATE request, we are a full duplex device.
- Improve behaviour of the CC2531 USB stick when there is no
  process on the host to read IN data. Basically, we adopt the
  CC2538 approach and we only send data when a DTE is present
Merge pull request #199 from g-oikonomou/cc253x-cdc-acm-line-state-fix

Improve handling of CDC ACM line state
CC2538 Documentation typo and grammar fixes
Change the InfoPage Location of the IEEE address
Merge pull request #202 from g-oikonomou/cc2538-minor-fixes

CC2538 IEEE address location and documentation fixes
tapdev: Fix using an uninitialized buffer
Add missing include

Fix log_message() undefined warning.
win32: Fix cygwin build

size_t is declared in sys/types.h