feat: enable usb + usb network adapters

This PR enables USB 2.0/3.0 devices in the kernel. It also, then,
enables the use of usb network adapters. This is useful in a baremetal
scenario where I'm using a usb-c to ethernet adapter for the node. The
options I explicitly enabled were: `CONFIG_USB`, `CONFIG_USB_XHCI_HCD`,
`CONFIG_USB_NET_DRIVERS`, and finally `CONFIG_USB_RTL8152`. All others
were added automatically by menuconfig.

Also added in a readme for how to make kernel config changes.

Signed-off-by: Spencer Smith <robertspencersmith@gmail.com>

kernel/README.md

@@ -0,0 +1,32 @@
+# Kernel
+
+## Customizing the kernel
+
+High-level notes:
+
+- In pkg.yaml, comment out the `build` and `install` steps
+- At the bottom of pkg.yaml, change the finalize step from
+
+```yaml
+finalize:
+  - from: /rootfs
+    to: /
+```
+
+to
+
+```yaml
+finalize:
+  - from: /
+    to: /
+```
+
+- Create a local image with `docker buildx build -t kernel --target kernel -f Pkgfile --load .`
+- Run the kernel image we created: `docker run --rm -it --entrypoint=/toolchain/bin/bash kernel`
+- Set path: `export PATH=/toolchain/bin:/bin`
+- Change to build dir: `cd /tmp/build/0`
+- Make changes to kernel settings with `make menuconfig` and save upon exiting
+- With the container still running, copy the config out to local disk: `docker cp $CONTAINER_ID:/tmp/build/0/.config config-amd64`
+- Revert your changes to pkg.yaml
+- Build and push a test image with `make USERNAME=rsmitty PUSH=true kernel`
+- PR upstream (when ready) and profit