How to build a standalone application of uboot with multiple source files

[Jojan]

I am building a stand alone ubbot applications  ( using  example/standalone/ hello_world) as a reference.My applications has multiple source files . 
In the stand alone application Makefile has the following line
COBJS   := $(ELF:=.o)
I added my additional files as follows
COBJS   := $(ELF:=.o) a.o b.o
How ever this results in undefined reference to to the symbols defined in a.c and  b.c
a.o and b.o are not added in the the linker inputs