qemu/machine: rename QEMU_EXTRAOPTIONS for consistency

qemu/machine: rename QEMU_EXTRAOPTIONS for consistency

The per-tune qemu options variable is QEMU_EXTRAOPTIONS_${TUNE_PKGARCH},
but this doesn't follow the pattern of all of the other tune-specific
variables in the machine configuration which is VARIABLE:tune-[name].

Rename QEMU_EXTRAOPTIONS_${TUNE_PKGARCH} to
QEMU_EXTRAOPTIONS:tune-${TUNE_PKGARCH} for consistency.

Note that this will mean that BSPs need to update any assignments of
this variable.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>