eal: add function to release internal resources

eal: add function to release internal resources

This commit adds a new function rte_eal_cleanup().
The function serves as a hook to allow DPDK to release
internal resources (e.g.: hugepage allocations).

This function allows DPDK to become more like an ordinary
library, where the library context itself can be initialized
and cleaned up by the application.

The rte_exit() and rte_panic() functions must be considered,
particularly if they should call rte_eal_cleanup() to release any
resources or not. This patch adds the cleanup to rte_exit(),
but does not clean up on rte_panic(). The reason to not clean
up on panicing is that the developer may wish to inspect the
exact internal state of EAL and hugepages.

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Vipin Varghese <vipin.varghese@intel.com>