grub: replace string option for filesystem selection by booleans

grub: replace string option for filesystem selection by booleans

In 1cece2813bf635ae29e9a8287322439b9c5992f7 (grub: add option to
configure the list of supported filesystems), we introduced the
BR2_TARGET_GRUB_FS_SUPPORT option which allows to provide a
space-separated list of filesystems that Grub should support.

However, it turns out that this not very practical, because the
iso9660 filesystem logic in Buildroot should force the ISO9660 support
to be enabled in Grub, which is not easy to do with a string option.

Therefore, this patch changes this option from a string option to a
list of boolean option, one per filesystem supported.

A few useful details:

 - Since Grub legacy is dead, the list of filesystem, and therefore
   the number of options, will not grow.

 - We have only added options for filesystems that are likely to be
   used in an embedded Linux context. Filesystems such as VSTAfs,
   Minix, UFS2 or FFS2 are not supported.

 - There is no need to add some Config.in.legacy support for the
   previous option, since it was added after Buildroot 2013.11, and
   was therefore never part of an official Buildroot release.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>