kconfig/streamline-config.pl: Fix parsing Makefile with variables

Thomas Lange reported that when he did a 'make localmodconfig', his
config was missing the brcmsmac driver, even though he had the module
loaded.

Looking into this, I found the file:
drivers/net/wireless/brcm80211/brcmsmac/Makefile
had the following in the Makefile:

MODULEPFX := brcmsmac

obj-$(CONFIG_BRCMSMAC)  += $(MODULEPFX).o

The way streamline-config.pl works, is parsing all the
 obj-$(CONFIG_FOO) += foo.o
lines to find that CONFIG_FOO belongs to the module foo.ko.

But in this case, the brcmsmac.o was not used, but a variable in its place.

By changing streamline-config.pl to remember defined variables in Makefiles
and substituting them when they are used in the obj-X lines, allows
Thomas (and others) to have their brcmsmac module stay configured
when it is loaded and running "make localmodconfig".

Reported-by: Thomas Lange <thomas-lange2@gmx.de>
Tested-by: Thomas Lange <thomas-lange2@gmx.de>
Cc: Arend van Spriel <arend@broadcom.com>
Cc: stable@vger.kernel.org
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>