boost.mk: allow building both static and shared

Use BR2_SHARED_STATIC_LIBS for building both static and shared boost libs.
The static libs are copied to the staging directory. Only the shared libs are
copied to the rootfs image.

[Thomas:
 - remove empty else clause, and instead add a comment that explains
   why we're doing something only for the BR2_STATIC_LIBS=y and
   BR2_SHARED_LIBS=y.
 - remove trailing backslash at the end of the BOOST_CONF_OPTS
   definition.]

Signed-off-by: Jonathan Ben Avraham <yba@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/boost/boost.mk

@@ -89,9 +89,15 @@ endif
 BOOST_OPTS += toolset=gcc \
 	     threading=multi \
 	     abi=$(BOOST_ABI) \
-	     variant=$(if $(BR2_ENABLE_DEBUG),debug,release) \
-	     link=$(if $(BR2_STATIC_LIBS),static,shared) \
-	     runtime-link=$(if $(BR2_STATIC_LIBS),static,shared)
+	     variant=$(if $(BR2_ENABLE_DEBUG),debug,release)
+
+# By default, Boost build and installs both the shared and static
+# variants. Override that if we want static only or shared only.
+ifeq ($(BR2_STATIC_LIBS),y)
+BOOST_OPTS += link=static runtime-link=static
+else ifeq ($(BR2_SHARED_LIBS),y)
+BOOST_OPTS += link=shared runtime-link=shared
+endif
 
 ifeq ($(BR2_PACKAGE_BOOST_LOCALE),y)
 ifeq ($(BR2_TOOLCHAIN_USES_UCLIBC),y)