zcip: Add environment variable for overriding log functionality

function                                             old     new   delta
bb_logenv_override                                     -      70     +70
packed_usage                                       29969   30033     +64
zcip_main                                           1426    1431      +5
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(add/remove: 2/0 grow/shrink: 2/0 up/down: 139/0)             Total: 139 bytes

Signed-off-by: Michel Stam <m.stam@fugro.nl>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>