RTEMS: allocate space for POSIX keys which are required by current RTEMS shell version.

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
RTEMS: actual RTEMS git sources do not provide CONFIGURE_NUMBER_OF_DRIVERS define.

There is only one version of table - actual runtime version.
The read-only table with initialization data is not present
and cannot be used for to compute number of entries of config
harcoded drivers set (CONFIGURE_NUMBER_OF_DRIVERS).

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>