buildroot: update to 2018.05-rc1

qemu: update to v2.12.0

The Buildroot update fixes #28 which broke the build on Ubuntu 18.04,
which had error message:

    conflicting types for 'copy_file_range'

We shouldn't normally update to -rc tags, but it fixes the build and I can't
be bothered to bisect further, when 2018.05 comes we will just update again.

The qemu update is kind of random, and I shouldn't have done it at the same
time, but so be it. I did it because I noticed the build was broken while
trying to fix .gitignores on QEMU.

Author: cirosantilli

README.adoc

@@ -9,15 +9,15 @@
 :toclevels: 6
 :toc-title:
 
-Run one command, get a QEMU or gem5 Buildroot BusyBox virtual machine built from source with several minimal Linux kernel 4.16 module development example tutorials with GDB and KGDB step debugging and minimal educational hardware models. "Tested" in x86, ARM and MIPS guests, Ubuntu 17.10 host.
+Run one command, get a QEMU or gem5 Buildroot BusyBox virtual machine built from source with several minimal Linux kernel 4.16 module development example tutorials with GDB and KGDB step debugging and minimal educational hardware models. "Tested" in x86, ARM and MIPS guests, Ubuntu 18.04 host.
 
 toc::[]
 
 == Getting started
 
 === Getting started Ubuntu
 
-This is the most native setup, and therefore the best one if you are on one of the supported Ubuntu: 16.04 or 17.10. We will try to keep up with both the latest LTS and release.
+This is the most native setup, and therefore the best one if you are on one of the supported Ubuntu: 16.04 or 18.04.
 
 Reserve 12Gb of disk and run:
 
@@ -97,6 +97,11 @@ hello /root/.profile
 
 All available modules can be found in the link:kernel_module/[`kernel_module` directory].
 
+We will try to support the following Ubuntu versions at least:
+
+* if the the latest release is an LTS, support both the latest LTS and the previous one
+* otherwise, support both latest LTS and the latest non-LTS
+
 [[docker]]
 === Getting started Docker
 
@@ -2385,7 +2390,7 @@ cd ..
 git branch "buildroot-2017.08-linux-${last_mainline_revision}"
 git add .
 # And update the README to show off.
-git commit -m "Linux ${next_mainline_revision}"
+git commit -m "linux: update to ${next_mainline_revision}"
 # Test the heck out of it, especially kernel modules and GDB.
 ./run
 git push