We were duplicating the make args a few times so make a little ARGS
variable.
Default to the /lib/modules/$(uname -r) installed kernel source if
SK_KSRC isn't set.
And only try a sparse build that can fail if we can execute the sparse
command.
Signed-off-by: Zach Brown <zab@versity.com>
Reviewed-by: Mark Fasheh <mfasheh@versity.com>