kbuild: Fix size_append issue for bzip2/lzma kernel

The Makefile.lib will call "echo -ne" to append uncompressed kernel size to
bzip2/lzma kernel image.
The "echo" here depends on the shell that /bin/sh pointing to.
On Ubuntu system, the /bin/sh is pointing to dash, which does not support
"echo -e" at all. Use /bin/echo instead of shell echo should always be safe.

Signed-off-by: Alek Du <alek.du@intel.com>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>