package/qemu: add support for FDT

FDT is the Flat Device Tree, and allows QEMU to pass DTs to the VMs.

Fixes:
http://autobuild.buildroot.net/results/7e8/7e8c6fecbb22e8835269171b8bfe6d5bda300dcc/
http://autobuild.buildroot.net/results/5e0/5e0ec37894d3d88962fb0fee9a3753b83fe18b71/
http://autobuild.buildroot.net/results/936/936c565b6dc883beea4fac47c2cb2b47d3e84f66/
http://autobuild.buildroot.net/results/2ab/2abe26be5be5e72d6e166dd9b63b61523f63e2f5/



And many more.

[Peter: add autobuilder references]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>