Do not attempt to use the systemwide libfdt

U-Boot bundles a patched copy of libfdt, so it's wrong to attempt to
include it <like/this>. This breaks the build for me when I have dtc
fully installed in my host -- as happened earlier tonight with
Buildroot, for example.

There are several other occurrences throughout the code where '<libfdt'
matches. I'm not modifying these because I have no clue why the
<systemwide> include style is being used -- IMHO wrongly.

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>