Scan directories that store PSU images on start

When the service starts, scan the directories that store PSU images,
including the built-in images, and the saved images during PSU update.

When the

Scan directories that store PSU images on start

When the service starts, scan the directories that store PSU images,
including the built-in images, and the saved images during PSU update.

When the scanned image is different than the running images, create
activation/version object;
When the scanned image is the same as the running images, update the
version object's path to indicate the PSU image path, so it could be
used for future update in case a PSU is replaced with a different
software.

Tested: On Witherspoon, fake create a dummy PSU image with a different
version than running PSU, verify a new object is created on
restart;
fake creating a dummy PSU image with a same version as a running
PSU, verify no new object is created, but the "Path" property is
set to the PSU image directory.

Signed-off-by: Lei YU <mine260309@gmail.com>
Change-Id: I860b978250a718eb82d948a1c88bd8f41bb2b2e3

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