crosfw: Use /tmp as output directory by default
crosfw: Use /tmp as output directory by default

Rather than pollute the U-Boot directory, put output files into /tmp.
Add an option to select a different directory if desired.

BUG=none
TEST=manual
crosfw -b sandbox -w
(puts files in /tmp/sandbox)

Set OUT_DIR = '/tmp/u-boot' in ~/.crosfw

crosfw -b sandbox -w
(puts files in /tmp/u-boot/sandbox)

Change-Id: I16c517c660df5234585091de1c8206695b57cf40
Signed-off-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/62545
Reviewed-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>

bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite