build objects in separate directory to fix caching

Setting KCFLAGS="-ffunction-sections -fdata-sections" causes make to
invalidate all the kernel objects, resulting in all the objects getting
rebuilt on the next pass, thus no build caching.

To fix that, build the objects in a separate directory (obj) for normal
builds, and another separate directory (obj2) for the builds with added
cflags.
Merge pull request #34 from jpoimboe/cache-fix

build objects in separate directory to fix caching