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.

(From OE-Core rev: 77497dbdca92ab4d6386a071bc281c42a7e8a14b)

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>

meta/classes-recipe/kernel.bbclass

@@ -336,7 +336,7 @@ kernel_do_transform_bundled_initramfs() {
 do_transform_bundled_initramfs[dirs] = "${B}"
 
 python do_package:prepend () {
-    os.environ['STRIP'] = d.getVar('KERNEL_STRIP')
+    d.setVar('STRIP',  d.getVar('KERNEL_STRIP').strip())
 }
 
 python do_devshell:prepend () {