Modular grub

Now, both the i386-pc and the amd64 UEFI GRUB images are modular.
This means, that startup is faster and that all modules (even those
that cannot be embedded) can be used.

The embedded configfile has been simplified since conditionals are not
supported by the simple command parser of the rescue shell, without
sacrificing any functionality.

Also, the main build script now copies the *.mod and *.lst files for
UEFI GRUB to the ISO.

Also added a remark to README.md to be careful when running `grml-live` directly from the checkout. 

Both GRUB images have been tested on a freshly built grml96-small image.