mk: remove variables identical to config ones

mk: remove variables identical to config ones

CONFIG_RTE_BUILD_SHARED_LIB and CONFIG_RTE_BUILD_COMBINE_LIBS does not
have quotes in their values (only y or n). That's why the variables
RTE_BUILD_SHARED_LIB and RTE_BUILD_COMBINE_LIBS are always identical to
their CONFIG_ counterpart, and are useless.
In order to have consistent naming of config options in the makefiles,
these options are removed and the "CONFIG_ prefixed" variables are used.

Fixes: e25e4d7ef16b ("mk: shared libraries")
Fixes: 4d3d79e7a5c6 ("mk: combined library")

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>