We try to maintain the minimal set of headers required to build, as the full
set of files from MdePkg is quite large (10MB at the time of writing). To do
this when performing an upgrade, execute the following:

# Generate list of the headers needed to build
cp -r ../vendor/edk2/dist/MdePkg/Include sys/contrib/edk2
cd lib/libefivar
make
pushd `make -V .OBJDIR`
cat .depend*.o | grep sys/contrib | cut -d' ' -f 3 |
    sort -u | sed -e 's=/full/path/sys/contrib/edk2/==' > /tmp/xxx
popd

# Merge the needed files
cd ../../sys/contrib/edk2
svn revert -R .
for i in `cat /tmp/xxx`; do
    svn merge -c VendorRevision svn+ssh://repo.freebsd.org/base/vendor/edk2/dist/MdePkg/$i $i
done
svn merge -c VendorRevision \
    svn+ssh://repo.freebsd.org/base/vendor/edk2/dist/MdePkg/MdePkg.dec MdePkg.dec