zram-swap: compression algorithm configuration option

zram-swap: compression algorithm configuration option

Compression algorithms for zram are provided by kernel crypto API, could
be any of [lzo|zl4|deflate|<some_more>] depending on kernel modules.
Compress algo for zram-swap could be defined via 'zram_comp_algo' option
in 'system' section of '/etc/config/system' file, or via cli (for e.x.
with 'uci set system.@System[0].zram_comp_algo=lz4 && uci commit
system'). check available algo's via 'cat /sys/block/zram0
/comp_algorithm'

Signed-off-by: Emil Muratov <gpm@hotplug.ru>
(backported from b9e89adfb7881806d01e3bd259852e352f3b3ce8)