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Set the directory to look for the kernel source when building external
modules.
-The directory can be specified in several ways:
-1) Use "M=..." on the command line
-2) Environment variable KBUILD_EXTMOD
-3) Environment variable SUBDIRS
-The possibilities are listed in the order they take precedence.
-Using "M=..." will always override the others.
+Setting "M=..." takes precedence over KBUILD_EXTMOD.
KBUILD_OUTPUT
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