LU-13783 build: Fix LB_LINUX_VERSION rule for v5.8

LU-13783 build: Fix LB_LINUX_VERSION rule for v5.8

Since Commit 20b1be595282 ("kbuild: fix single target builds for
external modules") in v5.8-rc7, the LB_LINUX_VERSION autoconf
rule doesn't work.

I don't know exactly why, but it can be fixed by setting "makerule" to
an empty string.

Passing the path to the directory in $makerule is unnecessary as
LB_LINUX_COMPILE_IFELSE, which LB_LINUX_TRY_MAKE eventually calls,
passes "$MODULE_TARGET=$PWD/build" which has the required effect.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I2b381d3546aaa0f365328a1319b2d4f145f33eeb
Reviewed-on: https://review.whamcloud.com/40371
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>