[ipxe-devel] [ipxe/ipxe] fix EMBED: don't eagerly assign over non-empty value (#115)

[realtime-neil]

> What problem is this actually solving?

Allow me to demonstrate:

```
$ echo && cat Makefile.bad && echo && EMBED=foobar make -f Makefile.bad && echo && cat Makefile.good && EMBED=foobar make -f Makefile.good 

EMBED := $(EMBEDDED_IMAGE)

.PHONY: all
all:
	echo "EMBEDDED_IMAGE: $(EMBEDDED_IMAGE)"
	echo "EMBED: $(EMBED)"


echo "EMBEDDED_IMAGE: "
EMBEDDED_IMAGE: 
echo "EMBED: "
EMBED: 

EMBED ?= $(EMBEDDED_IMAGE)

.PHONY: all
all:
	echo "EMBEDDED_IMAGE: $(EMBEDDED_IMAGE)"
	echo "EMBED: $(EMBED)"

echo "EMBEDDED_IMAGE: "
EMBEDDED_IMAGE: 
echo "EMBED: foobar"
EMBED: foobar
```

When the definition of the shell variable `EMBED` precedes `make`, `Makefile.good` shows the effect and `Makefile.bad` does not.

> Any variables specified in the `make` command line will override assignments within the Makefile. Building with `make EMBED=...` works with no problems already.

You're not wrong: `make EMBED=...` works. What doesn't work is `EMBED=... make` or any combination of exporting a shell variable for later consumption by a `make` invocation. For reasons that I have yet to dig out of the GNU Make manual, consuming that variable definition only works when it appears as a argument to the `make` command.

Strictly speaking, it's not incorrect to accept only variable assignments that happen to follow the command, but it will confuse others (like me) that assume the `EMBED` should behave like a shell variable when the immediate assignment prevents exactly that.


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