bitbake.conf: add STAMPS_DIR for constructing STAMP

bitbake.conf: add STAMPS_DIR for constructing STAMP

Add STAMPS_DIR for constructing STAMP, the defination of STAMP is:

STAMP = "${TMPDIR}/stamps/${MULTIMACH_TARGET_SYS}/${PN}/${EXTENDPE}${PV}-${PR}"

We can only change the TMPDIR if we want to change the STAMP's location,
but the bb_cache.dat would be regenerated if TMPDIR changes, so add
STAMPS_DIR for constructing it, and add it to the BB_ENV_EXTRAWHITE,
this is very usefull for the "bitbake -S", since then it can be run by:

STAMPS_DIR=<path> bitbake -S <recipe>

which will avoid putting the stamps to ${TMPDIR}/stamps.

BTW, break the too long BB_ENV_EXTRAWHITE into several lines.

[YOCTO #1659]

(From OE-Core rev: ce732c04b3ac06633e20efa8799c4189abfd41b3)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>