package/libgeos: use specific bug instead of BR2_nios2 to disable package

package/libgeos: use specific bug instead of BR2_nios2 to disable package

Let's use the _BUG_ form for disabling this package instead of BR2_nios2
architecture as we already use it for other packages.

Propagate this dependency to postgis. Also add the missing dependency on
bug 21464 to postgis.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Arnout:
 - put 27597 after 21464 instead of before it;
 - propagate dependency to postgis;
 - mention the bugs in the comments.
]