mk: fix build of shared library with libbsd

mk: fix build of shared library with libbsd

When building DPDK with "make" and options
	CONFIG_RTE_USE_LIBBSD=y
and
	CONFIG_RTE_BUILD_SHARED_LIB=y
libbsd was not linked, resulting in compilation errors:
	undefined reference to `strlcpy'

The link option -lbsd is added in a common place for both
Linux apps and libs.
It is used in app linkage via EXECENV_LDLIBS,
and in lib linkage via the added variable EXECENV_LDLIBS-y.

Fixes: 5364de644a4b ("eal: support strlcpy function")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>