Add -D{board.variant} to build.extra_flags of PCA1000X softdevices

[oyooyo]

The build.extra_flags for PCA1000X boards include -D{board.variant}:
PCA1000X.build.extra_flags=-DNRF51 -D{board.variant}
However, this extra flag is currently not also part of the build.extra_flags of the PCA1000X softdevices:

PCA1000X.menu.softdevice.s110.build.extra_flags=-DNRF51 -DS110 -DNRF51_S110
[...]
PCA1000X.menu.softdevice.s130.build.extra_flags=-DNRF51 -DS130 -DNRF51_S130

Because of this, -D{board.variant} will actually only be used on PCA1000X boards when selecting softdevice "none".
On PCA1000X boards that require hardware flow control for UART communication (like the PCA10000 and PCA10001), this for example leads to the strange effect that serial port communication currently only works when selecting softdevice "none".

[sandeepmistry]

@oyooyo these changes look ok to me, could you please also update the Travis CI file to leverage the new variant, see: https://github.com/sandeepmistry/arduino-nRF5/blob/master/.travis.yml#L30-L31

You'll just need a new line.

[dlabun]

It looks like changes from PR#226 to add the new board is also included in this PR... Could you remove them?

I'm dismissing my review (Sandeep already caught this and commented on #226)

[oyooyo]

I believe I was successful at removing the duplicate commit, so this PR should be okay now.