add GRUB_WINDOWS_EXTRA_DIST to allow shipping runtime files

add GRUB_WINDOWS_EXTRA_DIST to allow shipping runtime files

Not all toolkits provide static libraries. This patch enables creation of self
contained distribution that does not require pre-existing runtime libraries.
Intended usage is

export GRUB_WINDOWS_EXTRA_DIST="/path/to/liblzma.dll /path/to/libintl.dll"
make
make windowszip

As those libraries and locations are dependent on toolchain in use, trying
to autodetect them is likely impossible. So just provide a simple way to
package everything in one step.

Also remove $(windowsdir) after ZIP was created same as other "make dist"
targets.