efd: fix build by removing dependency to libmath

efd: fix build by removing dependency to libmath

When we compile the dpdk with:
  CONFIG_RTE_LIBRTE_EFD=y
  CONFIG_RTE_LIBRTE_NFP_PMD=n
  CONFIG_RTE_LIBRTE_THUNDERX_NICVF_PMD=n
  CONFIG_RTE_LIBRTE_SCHED=n
  CONFIG_RTE_LIBRTE_METER=n

The linker gives the following error:
  lib/librte_efd.a(rte_efd.o): In function `rte_efd_create':
  lib/librte_efd/rte_efd.c:560: undefined reference to `log2'
  collect2: error: ld returned 1 exit status

This is because the '-lm' is missing in mk/rte.app.mk.

An alternative, which is proposed by this patch, is to use the compiler
builtin rte_bsf32() to process log2 instead of the libmath log2() that
requires to include math.h and link with -lm.

Fixes: 56b6ef874f80 ("efd: new Elastic Flow Distributor library")

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>