scripts/dtc: add missing libfdt and fixups to import script

The following script is used to import dtc upstream
into kernel. It was missing import of dtc/libfdt, with
this commit it will include the same.

Also, currently in arch and driver code that needs early access to
the flattened device tree it is necessary to add specific CFLAGS
so that when scripts/dtc/libfdt/libfdt.h is included the C
preprocessor is able to locate the libfdt versions of libfdt_env.h
and fdt.h without generating an error. So, this script replaces
angular brackets with quotation used to include header files
fdt.h and libfdt_env.h

Signed-off-by: Gaurav Minocha <gaurav.minocha.os@gmail.com>
[robh: reword commit headline and add 'git add']
Signed-off-by: Rob Herring <robh@kernel.org>