driver: Allow environment variables to be passed as parameters

Presubmit: https://travis-ci.com/nathanchance/continuous-integration/builds/103470578

[skip ci]

Signed-off-by: Nathan Chancellor <[email protected]>

Author: nathanchance

driver.sh

@@ -5,6 +5,7 @@ set -eu
 setup_variables() {
   while [[ ${#} -ge 1 ]]; do
     case ${1} in
+      "AR="*|"ARCH="*|"CC="*|"LD="*|"REPO="*) export "${1?}" ;;
       "-c"|"--clean") cleanup=true ;;
       "-j"|"--jobs") shift; jobs=$1 ;;
       "-j"*) jobs=${1/-j} ;;

usage.txt

@@ -4,14 +4,23 @@ Script description: Build a Linux kernel image with Clang and boot it
 
 Environment variables:
   The script can take into account specific environment variables, mostly used
-  with Travis.  They can be invoked either via 'export VAR=<value>;
-  ./driver.sh' OR 'VAR=value ./driver.sh'
+  with Travis.  They can be invoked in one of three ways:
+    $ export VAR=<value> && ./driver.sh
+    $ VAR=value ./driver.sh
+    $ ./driver.sh VAR=value
 
+  AR:
+      If no AR value is specified, the script will attempt to set AR to
+      llvm-ar-9, llvm-ar-8, llvm-ar-7, llvm-ar, then ${CROSS_COMPILE}ar
+      if they are found in PATH.
   ARCH:
       If no ARCH value is specified, arm64 is the default. Currently, arm32_v7,
       arm32_v6, arm32_v5, arm64, ppc64le and x86_64 are supported.
+  CC:
+      If no CC value is specified, the script will attempt to set CC to
+      clang-9, clang-8, clang-7, then clang if they are found in PATH.
   LD:
-      If no LD value is specified, ${CROSS_COMPILE}-ld is used. arm64 only.
+      If no LD value is specified, ${CROSS_COMPILE}ld is used. arm64 only.
   REPO:
       Nicknames for trees:
         linux (default)