Correct value of BLE.connected()

[sandeepmistry]

As discussed in #507.

Now it will check:

• if in peripheral mode and if a central is connected
• if in central mode and if a peripheral is connected

I've also reverted https://github.com/01org/corelibs-arduino101/commit/a2df1795ac275ac48485329dc0697ab2324715bf from PR #454 as the change it no longer required with https://github.com/01org/corelibs-arduino101/commit/58c323e278a510e6e6ec2d0c4f29de69a0c11384.

cc/ @jimaobian

[SidLeung]

@sgbihu please review the code change as a solution to,

https://github.com/01org/corelibs-arduino101/issues/507#issuecomment-292660771

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