build/envkernel: Set CBUILD env variable for abuild rootpkg (!1779)

abuild depends on the gcc binary in order to define a default CBUILD value.
When using an alternative gcc version (e.g. envkernel.sh with gcc6), the
gcc binary is not installed by the envkernel.sh script.

This change sets the CBUILD env variable so that abuild doesn't need to
depend on the gcc package.

pmb/build/envkernel.py

@@ -158,6 +158,7 @@ def run_abuild(args, pkgname, arch, apkbuild_path, kbuild_out):
     # Create the apk package
     env = {"CARCH": arch,
            "CHOST": arch,
+           "CBUILD": args.arch_native,
            "SUDO_APK": "abuild-apk --no-progress"}
     cmd = ["abuild", "rootpkg"]
     pmb.chroot.user(args, cmd, working_dir=build_path, env=env)