kernel: clean up XATTR config symbols

kernel: clean up XATTR config symbols

Extended attributes are required for overlayfs and have hence been long
ago enabled for jffs2, but should be enabled unconditionally for all
other filesystems which may potentially serve as overlayfs' upper
directory. Previously it was inconsistently added in multiple targets.
Add symbols to generic kernel config and remove all *_XATTR symbols
from target configs.

Signed-off-by: Paul Spooren <mail@aparcar.org>
[keep things as they are for squashfs, improve commit message]
Signed-off-by: Daniel Golle <daniel@makrotopia.org>