Rename buildroot_config_fragment_* to just br2_*

Add br2_local to persistently store new br2 options.

build

@@ -1,10 +1,10 @@
 #!/usr/bin/env bash
 set -eu
 arch=x86_64
-rm -f buildroot_config_fragment_cli
-touch buildroot_config_fragment_cli
+rm -f br2_cli
+touch br2_cli
 configure=true
-config_fragments='buildroot_config_fragment buildroot_config_fragment_cli'
+config_fragments='br2 br2_cli'
 extra_make_args=''
 gem5=false
 j="$(($(nproc) - 2))"
@@ -18,7 +18,7 @@ while getopts 'a:c:Cgj:i:kK:lp:qS:v' OPT; do
       arch="$OPTARG"
     ;;
     c)
-      echo "$OPTARG" >> buildroot_config_fragment_cli
+      echo "$OPTARG" >> br2_cli
     ;;
     C)
       configure=false
@@ -57,6 +57,9 @@ while getopts 'a:c:Cgj:i:kK:lp:qS:v' OPT; do
 done
 shift $(($OPTIND - 1))
 extra_make_args="$extra_make_args $@"
+if [ -f br2_local ]; then
+  config_fragments="$config_fragments br2_local"
+fi
 case "$arch" in
   x86_64)
     defconfig=qemu_x86_64_defconfig
@@ -77,7 +80,7 @@ if "$gem5"; then
   # Networking was not working, so disable it to speed things up.
   post_script_args="$post_script_args -n"
 else
-  config_fragments="$config_fragments buildroot_config_fragment_qemu"
+  config_fragments="$config_fragments br2_qemu"
 fi
 root_dir="$(pwd)"
 buildroot_dir="${root_dir}/buildroot"