Failed to install SONiC base image (sonic-generic.bin) with ONIE in our x86 switch

[sonicyao1985]

In the following commit:
cc7f150#diff-028739c9bdaa77f69d654ce551a93cec

In file installer/x86_64/install.sh, the following line is introduced:
source platforms/$onie_platform

Since we don't need extra cmd for our own platform, I didn't add a file under the "platforms" directory.
Thus the installing failed due to absence of the platform file.

I think it is better to check the existence of file before source it.

[lguohan]

addressing this in #37