util/grub-install-common: Confirm directory creation in grub_install_mkdir_p()

util/grub-install-common: Confirm directory creation in grub_install_mkdir_p()

Because grub_util_mkdir() is implemented to not return a value on any
platform, grub_instal_mkdir_p() can test for success by confirming that
the directory requested exists after attempting to create it, otherwise
it should fail with an error and exit.

While fixing this, a flaw in the logic was shown, where the first match
of the path separator, which almost always was the first character in
the path (e.g. /boot/grub2) would result in creating a directory with an
empty name (i.e. ""). To avoid that, it should skip the handling of the
path separator where p is pointing to the first character.

Signed-off-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>