Allow customizing the checkpatch command

Provide an example use-case with DPDK checkpatches.sh in README.md

Signed-off-by: Haggai Eran <haggaie@nvidia.com>

Author: haggaie

README.md

@@ -35,6 +35,25 @@ jobs:
       uses: webispy/checkpatch-action@master
 ```
 
+For using a custom checkpatch script, pass the `CHECKPATCH_COMMAND` environment
+variable. For example, for DPDK's `checkpatches.sh` script use:
+
+```yml
+name: checkpatch review
+on: [pull_request]
+jobs:
+  my_review:
+    name: checkpatch review
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - name: Run DPDK checkpatches.sh review
+      uses: webispy/checkpatch-action@master
+      env:
+        DPDK_CHECKPATCH_PATH: /usr/bin/checkpatch.pl
+        CHECKPATCH_COMMAND: ./devtools/checkpatches.sh
+```
+
 ## References
 
 ### checkpatch tool

review.sh

@@ -3,9 +3,13 @@
 # To debug the current script, please uncomment the following 'set -x' line
 #set -x
 
+if [[ -z "$CHECKPATCH_COMMAND" ]] ; then
+    CHECKPATCH_COMMAND="checkpatch.pl --no-tree"
+fi
+
 # Generate email style commit message
 PATCH_FILE=$(git format-patch $1 -1)
-PATCHMAIL=$(checkpatch.pl --no-tree $PATCH_FILE)
+PATCHMAIL=$($CHECKPATCH_COMMAND $PATCH_FILE)
 
 # Internal state variables
 RESULT=0