package.bbclass: fix strip and split logic

Marks original commit message and variable documentation state that stripping and splitting are independent of eachother, but package.bbclass ANDs the two INHIBIT flags to see which files can be stripped and/or split.

Original behaviour:

INHIBIT_PACKAGE_STRIP: no strip, no debug split
INHIBIT_PACAKGE_DEBUG_SPLIT: no strip, no debug split

Behaviour after this patch:

INHIBIT_PACKAGE_STRIP: no strip, no debug split
INHIBIT_PACKAGE_DEBUG_SPLIT: strip, no split

Signed-off-by: Koen Kooi <koen.kooi@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>