bitbake: toaster: check if file exist

bitbake: toaster: check if file exist

Buildinfohelper assumes that all files mentioned in
manifest exist in deploy/ directory, which is not always
the case. Toaster crashes with OSError trying to
call os.stat on non-existing file.

Checking if file exists before processing it should
fix this.

[YOCTO #10185]

(Bitbake rev: 54565e7ca84d2722a2454e7fa52cda564b28b527)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>