qemu: set default RAM to 256M for all machines

qemu: set default RAM to 256M for all machines

There was a discussion about what amount of RAM is appropriate for a
default; the outcome was that for now it is still 256M. Some qemu machine
definitions have however set this to 512M so for the sake of
treating all architectures fairly, they are reset back to 256M.

Also runqemu is adjusted to use 256M if QB_MEM is not set at all.

http://lists.openembedded.org/pipermail/openembedded-core/2019-August/285900.html

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>