The new 15.0 makefiles for hwlibs expect the ALT_DEVICE_FAMILY variable to be set to "soc_cv_av" or "soc_a10".
ALT_DEVICE_FAMILY ?= soc_cv_av
CFLAGS := -g $(OFLAG) -Wall -Werror -std=c99 $(MULTILIBFLAGS) -I$(HWLIBS_ROOT)/include -I$(HWLIBS_ROOT)/include/$(ALT_DEVICE_FAMILY) -D$(ALT_DEVICE_FAMILY)
ALL_HWLIBS_SRC = $(wildcard $(HWLIBS_ROOT)/src/hwmgr/*.c) $(wildcard $(HWLIBS_ROOT)/src/hwmgr/$(ALT_DEVICE_FAMILY)/*.c $(wildcard $(HWLIBS_ROOT)/src/utils/*.c))
On the CFLAGS, you need to add the soc_cv_av or soc_a10 directories to the includes with -I and define either -Dsoc_cv_av or -Dsoc_a10. The ALL_HWLIBS_SRC variable above finds all the .c files in all the directories.