Include ALL_MODULES in MKMODULESENV

[jtl]

Summary

While testing another change, I noticed that we don't include ALL_MODULES
in MKMODULESENV. It seems like we should do that when ALL_MODULES is
defined. (For example, r307082 relies on seeing the ALL_MODULES symbol
in a sub-make in the modules tree.)

Test Plan

With the change, the ALL_MODULES symbol made it to sub-makes in the modules tree.

[jtl]

bump

[imp]

This will just pass it in when it is defined in the kernel config file as a makeconfig option. Otherwise it passes through because it is in the environment, or it isn't defined. No?

[sjg]

ok