sm8550-sheng: build kernel with clang via KERNEL_COMPILER, not global export

The family built the kernel with clang by exporting CC="clang" and LLVM=1
globally. Those exports leak out of the kernel compilation into the
image-assembly chroot: the linux-headers postinst runs `make ... scripts`
(rebuilding scripts/basic/fixdep in the target rootfs), inherits LLVM=1, and
invokes clang - which is not installed in the image (the linux-headers package
Depends on gcc, not clang). The image build then dies with:

  make[1]: clang: No such file or directory
  /bin/sh: 1: clang: not found
  make[2]: *** [scripts/Makefile.host:114: scripts/basic/fixdep] Error 127
  dpkg: error processing package linux-headers-edge-sm8550-sheng (--configure)

Use the framework's first-class clang path instead: KERNEL_COMPILER="clang"
adds LLVM=1 only to the kernel make params, which run under `env -i`, so the
flag never escapes into the rootfs. The kernel is still built with clang;
the headers postinst rebuilds scripts with gcc (its declared dependency).

Signed-off-by: Igor Pecovnik <igor@armbian.com>

config/sources/families/sm8550-sheng.conf

@@ -26,5 +26,9 @@ case $BRANCH in
 
 esac
 
-export CC="clang"
-export LLVM=1
+# Build the kernel with clang/LLVM. Use KERNEL_COMPILER (scoped to the kernel
+# make, passed as LLVM=1 under `env -i`) rather than exporting CC/LLVM globally:
+# a global export leaks into the image-assembly chroot, where the linux-headers
+# postinst's `make scripts` then invokes clang (not installed in the rootfs -
+# the headers package Depends on gcc) and fails with "clang: not found".
+declare -g KERNEL_COMPILER="clang"