buildroot

qemu_ppc64_e5500_defconfig: convert quotes in readme.txt emulation example

Resolves an error in the way the bootlin toolchain-builder uses
the board/qemu/ppc64-e5500/readme.txt to generate the qemu test
command.

https://github.com/free-electrons/toolchains-builder/blob/master/build.sh#L186

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>