Makefile: fix parallel builds

This problem shows up with parallel builds only; it results in
somewhat cryptic error messages like

	$ JOBS=-j6 MAKEALL netstar
	Configuring for netstar board...
	arm-linux-ld: cannot find -lgeneric
	make[1]: *** [eeprom.srec] Error 1

A few boards (like netstar and voiceblue) need some libraries for
building; however, the board Makefile does not contain any such
dependencies which may cause problems with parallel builds. Adding
such dependencies is difficult as we would also have to provide build
rules, which already exist in the respective library Makefiles.

To solve this, we make sure that all libraries get built before the
board code.

Signed-off-by: Wolfgang Denk <wd@denx.de>