There is no initial_solution.manifest in the second build when
incremental rpm image generation, since the initial solution has been
skipped. So we should check it before cat it.
[YOCTO #3128]
(From OE-Core rev: ad17fa82a481ab3c9f17a8338ebad1eb07c0f9d8)
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>