kernel-module-split.bbclass: enhance objcopy command call for kernel compilation...

kernel-module-split.bbclass: enhance objcopy command call for kernel compilation with llvm

Using meta-clang for llvm kernel compilation previously defaulted to the gcc objcopy tool.
To improve flexibility and compatibility, $OBJCOPY is preferred over $HOST_PREFIXobjcopy
in the kernel-module-split.bbclass.

With $OBJCOPY already defined in bitbake.conf, the empty condition has been removed,
simplifying the invocation process.

Signed-off-by: lixiaoyong <lxy204899@163.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>