readme: multiple versions of a package in gem5

Author: cirosantilli

README.adoc

@@ -5191,6 +5191,26 @@ If none of those methods are flexible enough for you, create a new package as fo
 +
 if you make any changes to that package after the initial build: <<rebuild>>
 
+=== Build multiple versions of the same package
+
+If you are comparing two versions of have a package that takes considerable time to build, one on each branch, you may want to keep two builds around to make things even faster.
+
+One simple approach is to simply use two checkouts of this repository.
+
+While <<ccache>> does help with this, it still does not necessarily make compilation instantaneous due to configuration file reading.
+
+One alternative is to do:
+
+....
+./build KERNEL_MODULE_VERSION=mybranch
+....
+
+and now you can see that a new version of `kernel_module` was built and put inside the image:
+
+....
+ls out/x86_64/buildroot/build/kernel_module-mybranch
+....
+
 === BR2_TARGET_ROOTFS_EXT2_SIZE
 
 When adding new large package to the Buildroot root filesystem, it may fail with the message: