eal: add support for shared object drivers

eal: add support for shared object drivers

Add an option to specify libraries to be loaded before probing the PCI.

For instance, testpmd -d librte_pmd_xxx.so can be used to enable xxx driver
support on testpmd without any recompilation of testpmd.

Plugins are loaded before creating threads because we want the threads to
inherit any property that could be set while loading a plugin, such as iopl().

Signed-off-by: Damien Millescamps <damien.millescamps@6wind.com>
Signed-off-by: Jean-Mickael Guerin <jean-mickael.guerin@6wind.com>
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>