lm-sensors: fix static-only builds

This commit adds a patch to lm-sensors to support a new variable
BUILD_SHARED_LIB that allows to conditionally enable or disable the
build (and usage) of the libsensors shared library.

It also refactors the make variables in a variable called
LM_SENSORS_MAKE_OPT.

Fixes:

  http://autobuild.buildroot.org/results/06c/06c197263e4939d6adc4877e152f602a69df751d/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>