qemuimagetest: Use same image during sanity testing instead of copying a new image for each case
To reduce the time on sanity testing, we remove variable SHARE_IMAGE and use
a new variable TEST_SERIALIZE in local.conf. It is by default set to 1. Poky
will copy and boot the to-be tested image for only once. It will not remove
or kill the image and test cases will be serialized executed against the same
image. If it is set to 0, image is always be copied for each cases, which takes
much time. I had a experiment that latest qemuppc sato only takes 7 minutes to
finish 9 sanity test cases, which takes more than 20 minutes before.
I also removed sanity case "boot" from sato/sdk/lsb because the other cases for
these targets already cover the check point of "boot".
