mk: fix linking drivers in static apps

mk: fix linking drivers in static apps

The variable CONFIG_RTE_BUILD_SHARED_LIB was in rte.app.mk as
"RTE_BUILD_SHARED_LIB", which meant that none of the example apps linked
in the PMDs and just didn't work with any eth ports in any static builds.

This bug has been introduced in commit 3660cdf990:
  pcap: convert to use of PMD_REGISTER_DRIVER and fix linking

Link for l2fwd before patch:
  "... -Wl,--whole-archive -Wl,-lrte_kni -Wl,-lrte_timer -Wl,-lrte_hash
-Wl,-lrte_lpm -Wl,-lrte_power -Wl,-lrte_meter -Wl,-lrte_sched -Wl,-lm
-Wl,-lrt -Wl,--start-group -Wl,-lrte_kvargs -Wl,-lrte_mbuf -Wl,-lethdev
-Wl,-lrte_malloc -Wl,-lrte_mempool -Wl,-lrte_ring -Wl,-lrte_eal
-Wl,-lrte_cmdline -Wl,-lrt -Wl,-lm -Wl,-ldl -Wl,--end-group
-Wl,--no-whole-archive"

Link for l2fwd after patch:
  "...  -Wl,--whole-archive -Wl,-lrte_kni -Wl,-lrte_timer -Wl,-lrte_hash
-Wl,-lrte_lpm -Wl,-lrte_power -Wl,-lrte_meter -Wl,-lrte_sched -Wl,-lm
-Wl,-lrt -Wl,--start-group -Wl,-lrte_kvargs -Wl,-lrte_mbuf -Wl,-lethdev
-Wl,-lrte_malloc -Wl,-lrte_mempool -Wl,-lrte_ring -Wl,-lrte_eal
-Wl,-lrte_cmdline -Wl,-lrte_pmd_vmxnet3_uio -Wl,-lrte_pmd_virtio_uio
-Wl,-lrte_pmd_ixgbe -Wl,-lrte_pmd_e1000 -Wl,-lrte_pmd_ring -Wl,-lrt
-Wl,-lm -Wl,-ldl -Wl,--end-group -Wl,--no-whole-archive"

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>