build: add load average limit to reduce CPU overcommitment

For make and ninja based build systems, no new jobs are started if the load
average is greater than number_of_cores * 1.5.

config/functions

@@ -250,19 +250,26 @@ setup_toolchain() {
 
   # parallel
   if flag_enabled "parallel" "yes"; then
-    NINJA_OPTS="$NINJA_OPTS -j$CONCURRENCY_MAKE_LEVEL"
-    export MAKEFLAGS="-j$CONCURRENCY_MAKE_LEVEL"
+    NINJA_OPTS="-j$CONCURRENCY_MAKE_LEVEL"
+    MAKEFLAGS="-j$CONCURRENCY_MAKE_LEVEL"
   else
-    NINJA_OPTS="$NINJA_OPTS -j1"
-    export MAKEFLAGS="-j1"
+    NINJA_OPTS="-j1"
+    MAKEFLAGS="-j1"
   fi
 
   # verbose flag
   if flag_enabled "verbose" "no"; then
-    NINJA_OPTS="$NINJA_OPTS -v"
-    export MAKEFLAGS="$MAKEFLAGS V=1 VERBOSE=1"
+    NINJA_OPTS+=" -v"
+    MAKEFLAGS+=" V=1 VERBOSE=1"
   fi
 
+  # average load limit
+  if [ "${CONCURRENCY_LOAD}" != "0" ]; then
+    NINJA_OPTS+=" -l${CONCURRENCY_LOAD}"
+    MAKEFLAGS+=" -l${CONCURRENCY_LOAD}"
+  fi
+  export MAKEFLAGS
+
   case "$1:$2" in
     target:meson)
       export DESTIMAGE="target"

config/optimize

@@ -1,8 +1,6 @@
 # Linker hash-style is set to gnu via gcc default
 LD_OPTIM="-Wl,--as-needed"
 
-NINJA_OPTS=""
-
 TARGET_CPPFLAGS=""
 TARGET_CFLAGS="$TARGET_CFLAGS -Wall -pipe $PROJECT_CFLAGS"
 TARGET_CXXFLAGS="$TARGET_CFLAGS"

config/options

@@ -77,6 +77,7 @@ VERBOSE="${VERBOSE:-yes}"
 #  Try values between 1 and number of processor cores present.
 #  default: use all cores
 [ -z "${CONCURRENCY_MAKE_LEVEL}" ] && export CONCURRENCY_MAKE_LEVEL=$(nproc)
+[ -z "${CONCURRENCY_LOAD}" ] && export CONCURRENCY_LOAD=$(python3 -c "import os; print('%.2f' % (os.cpu_count() * 1.5))")
 
 # cache size for ccache
 # Set the maximum size of the files stored in the cache. You can specify a