Building EC was broken as the build output from the previous board was
not being cleaned. `BUILD` is now always set, defaulting to "build",
instead of only when `BOARD` is specified. For good measure, add it
to the clean command in case a custom path is used.
Fixes: 569321f9ac79 ("scripts: Set EC build dir")
Signed-off-by: Tim Crawford <tcrawford@system76.com>
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