buildroot: x86: Don't optimize for the corei7 variant

Joel reported in #120 that when passing 'init=/bin/bash' via the kernel
cmdline, there is a nice panic that happens on boot due to an invalid
opcode. Nick suggested that it was possible that the rootfs was compiled
with ISA extensions that aren't supported by QEMU's virtual CPU, which
is exactly the case.

When I initially created these images for myself, I selected
'CONFIG_BR2_x86_corei7=y' (as that's what I was using at the time),
which selects a bunch of ISA extensions[1]:

  config BR2_x86_corei7
      bool "corei7"
      select BR2_X86_CPU_HAS_MMX
      select BR2_X86_CPU_HAS_SSE
      select BR2_X86_CPU_HAS_SSE2
      select BR2_X86_CPU_HAS_SSE3
      select BR2_X86_CPU_HAS_SSSE3
      select BR2_X86_CPU_HAS_SSE4
      select BR2_X86_CPU_HAS_SSE42

I have never noticed a problem because I always use the '-cpu host
-enable-kvm' options, which makes QEMU's CPU emulate the host's (which
supported those extensions). QEMU's default cpu "qemu64" only supports
SSE3[2], which Buildroot's default target variant "nocona" supports as
well. Use that variant and rebuild the images so there are no more
issues with 'init=/bin/bash'.

Presubmit: https://travis-ci.com/nathanchance/continuous-integration/builds/99019841

[1]: https://github.com/buildroot/buildroot/blob/master/arch/Config.in.x86
[2]: https://github.com/qemu/qemu/blob/abd5fc4c862d/target/i386/cpu.c#L1488

[skip ci]

Closes: #120
Reported-by: Joel Stanley <[email protected]>
Suggested-by: Nick Desaulniers <[email protected]>
Signed-off-by: Nathan Chancellor <[email protected]>

Author: nathanchance

buildroot/x86_64.config

@@ -1,5 +1,4 @@
 BR2_x86_64=y
-BR2_x86_corei7=y
 BR2_OPTIMIZE_3=y
 BR2_KERNEL_HEADERS_4_4=y
 BR2_BINUTILS_VERSION_2_31_X=y