buildroot baseline: use cpio to find minimal image sizes

README.adoc

@@ -3992,9 +3992,13 @@ On the upstream Buildroot repo at 7d43534625ac06ae01987113e912ffaf1aec2302 we ru
 
 ....
 make qemu_x86_64_defconfig
-printf 'BR2_CCACHE=y\n' >>.config
+printf '
+BR2_CCACHE=y
+BR2_TARGET_ROOTFS_CPIO=y
+BR2_TARGET_ROOTFS_EXT2=n
+' >>.config
 make olddefconfig
-time make BR2_JLEVEL="$(nproc)"
+time env -u LD_LIBRARY_PATH make BR2_JLEVEL="$(nproc)"
 ....
 
 Time: 11 minutes, 7 with full ccache hits. Breakdown: 47% GCC, 15% Linux kernel, 9% uclibc, 5% host-binutils. Conclusions:
@@ -4013,6 +4017,13 @@ This is consistent with the fact that ccache reduces the build time only partial
 
 The instructions counts varied very little between the baseline and LKMC, so runtime overhead is not a big deal apparently.
 
+Size:
+
+* `bzImage`: 4.4M
+* `rootfs.cpio`: 1.6M
+
+Zipped: 4.9M, `rootfs.cpio` deflates 50%, `bzImage` almost nothing.
+
 ==== Benchmark gem5 build
 
 How long it takes to build gem5 itself on <<P51>>