Makefile: allow appending to LIB in sub-makefiles

The top Makefile and the SPL Makefile have lines like those:

ifneq ($(CONFIG_AM33XX)$(CONFIG_OMAP34XX)$(CONFIG_OMAP44XX)$(CONFIG_OMAP54XX),)
LIBS += $(CPUDIR)/omap-common/libomap-common.o
endif

ifeq ($(SOC),mx5)
LIBS += $(CPUDIR)/imx-common/libimx-common.o
endif

This should be done in the arch/CPU/SoC specific sub-makefiles to
keep the top Makefiles clean. This patch also allows adding of new
arch/CPU/SoC specific libraries in the future without touching
the top Makefiles.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>