Without doing this I can't reuse prebuilts generated from one directory in another
build directory which is really not comfortable for developers. Due to difference
on PWD variable, base hash becomes different for every sstate package.
So here PWD is added to the hase base whitelist, which makes most prebuilts reusable
now in a different build directory. However there still remains one issue about
do_populate_sysroot sstate package. Absolute path is encoded in this task which
then blocks reuse on them. Perhaps we need some redesign for do_populate_sysroot into
two stages, with all absolute path related operations moved to the 2nd stage. This
way then the 1st stage can be reused.
