Fix Coverity Scan build target

When PR #1271 was merged (commit c34c8db4), it:

 1. Created a new coverity-build target in src/Makefile
 2. Updated .github/workflows/coverity.yml to call make coverity-build
 3. Forgot to add coverity-build as an explicit target in the top-level Makefile

This commit also simplifies src/Makfile a lot for readability.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.github/workflows/coverity.yml

@@ -72,7 +72,7 @@ jobs:
       - name: Build applications for Coverity
         run: |
           export PATH=`pwd`/coverity/bin:$PATH
-          cov-build --dir cov-int make coverity-build
+          cov-build --dir cov-int make coverity
 
       - name: Submit results to Coverity Scan
         env: