Build the Linux kernel independently from Buildroot

This will allow for other types of root filesystems that don't rely on Buildroot
to be added and used in the future.

Propagate --verbose on all build scripts to see full GCC commands.

build-all: allow for neat subsets

also 9p share rootfs_overlay. TODO document.

configure

@@ -82,6 +82,8 @@ build-essential \
 coreutils \
 cpio \
 expect \
+gcc-aarch64-linux-gnu \
+gcc-arm-linux-gnueabi \
 git \
 moreutils \
 rsync \
@@ -93,8 +95,6 @@ wget \
   if "$gem5"; then
     pkgs="${pkgs} \
 ccache \
-gcc-aarch64-linux-gnu \
-gcc-arm-linux-gnueabi \
 libgoogle-perftools-dev \
 protobuf-compiler \
 python-dev \