nrf51822: Remove hardcoded path to NRF51 SDK

Now, the user needs to provides NRF51_SDK_PATH to the build system. He
can pass it via make command line:

	$ make NRF51_SDK_PATH=/path/to/sdk

Or put it inside Makefile.config (recommended way):

	$ echo "NRF51_SDK_PATH=/path/to/sdk" >> Makefile.config
	$ make

NRF51_SDK_PATH must point to the directory that contains `nrf51822`
path.

Currently, blessed only supports the SDK version 4.4.x.
nrf51822: Add support for other Nordic boards

Until now, only old versions of PCA10001 evaluation board were
supported. This patch allows the user to reassign those variables
in order to compile blessed to those boards.

This patch wasn't tested with other nRF51822 boards.
nrf51822: Add README.md
examples: Update README
WIP: blessed: Update README