loader: Ignore zero-sized ELF segments

Some ELF files have program headers that specify segments that
are of zero size. Ignore them, rather than trying to create
zero-length ROM blobs for them, because the zero-length blob
can falsely trigger the overlapping-ROM-blobs check.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Hua Yanghao <huayanghao@gmail.com>
Message-id: 1502116754-18867-3-git-send-email-peter.maydell@linaro.org
