package/openvmtools: introduce SUBDIR variable and update patches

The Open-vm-tools package is now distributed from GitHub and a new
subdirectory was introduced, which breaks the Buildroot process.

This patch adds the _SUBDIR variable in the Makefile and also updates
all distributed patches.

Fixes:

http://autobuild.buildroot.net/results/bfd10c8d454a2686ebb1c4afdb49c55e39a9dd67/



Signed-off-by: Karoly Kasza <kaszak@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>