kernel: add a config option for keeping the kallsyms table uncompressed, saving ~9kb kernel size after lzma on ar71xx

[john@phrozen.org: added to my upstream queue 30.12.2016]
lede-commit: e0e3509b5ce2ccf93d4d67ea907613f5f7ec2eed
Signed-off-by: Felix Fietkau <nbd@nbd.name>