kbuild: use cmd_shipped instead of cmd_copy

We already have cmd_shipped in scripts/Makefile.lib.
Use it rather than defining a new command cmd_copy.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
kbuild: move cmd_mkimage to scripts/Makefile.lib

Because cmd_mkimage is used in various subdirectories,
it seems reasonable to define it in scripts/Makefile.lib.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>