tools: mkimage: use pkg-config to discover libcrypto linker flags

LibreSSL's libcrypto.so has an indirect dependency to librt for clock_gettime()
on Linux.

Use pkg-config to portably discover the required linker flags.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

tools/mkimage/Makefile

@@ -31,8 +31,8 @@ define Host/Prepare
 endef
 
 define Host/Compile
-	$(MAKE) -C $(HOST_BUILD_DIR) defconfig   HOSTCFLAGS="$(HOST_CPPFLAGS) $(HOST_CFLAGS)" HOSTLDFLAGS="$(HOST_LDFLAGS)"
-	$(MAKE) -C $(HOST_BUILD_DIR) tools-only  HOSTCFLAGS="$(HOST_CPPFLAGS) $(HOST_CFLAGS)" HOSTLDFLAGS="$(HOST_LDFLAGS)"
+	$(MAKE) -C $(HOST_BUILD_DIR) defconfig   HOSTCFLAGS="$(HOST_CPPFLAGS) $(HOST_CFLAGS)" HOSTLDFLAGS="$(HOST_LDFLAGS) $$$$(pkg-config --static --libs libcrypto)"
+	$(MAKE) -C $(HOST_BUILD_DIR) tools-only  HOSTCFLAGS="$(HOST_CPPFLAGS) $(HOST_CFLAGS)" HOSTLDFLAGS="$(HOST_LDFLAGS) $$$$(pkg-config --static --libs libcrypto)"
 endef
 
 define Host/Install