grub: add option to configure the list of supported filesystems

Grub can be configured to enable or disable the filesystems supported
in the stage 2. In addition, there are filesystem specific stage
1.5. In order to save space and build time, we provide a new option
that allows to give a space-separated list of filesystems that Grub
should support.

We default to support FAT and ext2, since most of the other
filesystems are fairly unlikely to be used.

We use this option to:

 1) Pass --enable-<fs> or --disable-<fs>, which enables/disables the
    support of <fs> in Grub stage 2.

 2) Install only the stage 1.5 that correspond to the selected
    filesystems.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>