feat(qemu-system): use consistent "Make" variable name
This will allow us to use `$(QEMU_SYSTEM)` in env var substitutions when passing `//labgrid/toolchain/qemu-system:resolved` to `toolchains`.
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This will allow us to use `$(QEMU_SYSTEM)` in env var substitutions when passing `//labgrid/toolchain/qemu-system:resolved` to `toolchains`.
mentioned in commit 482e4e01