linux: Deselect all unconfigured compression options

linux: Deselect all unconfigured compression options

The LINUX_KCONFIG_FIXUP_CMDS are meant to deselect any compression
option that are not selected in the buildroot configuration. But it only
deselects the last one in the list instead of all of them because it
overwrites the LINUX_COMPRESSION_OPT_ variable instead of appending to
it. Only the last option set to that variable gets deselected.

This produces the warning:

.config:2216:warning: override: KERNEL_GZIP changes choice state

is emitted when buildroot runs olddefconfig when buildroot configures a
kernel with a custom config that has a different kernel compression
option set to what is configured in buildroot.

Accumulate all the deselected compression options instead of overwriting
them to ensure all non-selected options get deselected..

Signed-off-by: Cam Hutchison <camh@xdna.net>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>