kernel.bbclass: Use strip utility used for kernel build in do_package

kernel.bbclass: Use strip utility used for kernel build in do_package

os.environ does not pass this down to runstrip() function and in
strip_execs() its using STRIP bitbake variable to find the strip utility
to use. Since there might be a trailing whitespace in KERNEL_STRIP
remove that otherwise python is not able to launch it.
e.g.

FileNotFoundError: [Errno 2] No such file or directory: 'riscv64-yoe-linux-strip '

This is more evident when STRIP and KERNEL_STRIP are different utilities
e.g. when using clang as default toolchain but using gcc+binutils only for
kernel build.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>