kernelscripts: Expose SHA1 value of the kernel tip

The SHA1 value associated with the binaries that are generated
is required to provide uniqueness between binaries that are
generated across various verification runs for the same
SI. Thus, export this information into the ship_prebuilt
directory so that it is available when the generated
binaries need to be uploaded to a filer.

Change-Id: I9a0e58230d6219b031f0617e4333be0ddd15e5b7
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>