rootdir: etc: change max readahead size to 512K in init

Change the VM_MAX_READAHEAD value from the default 128K
to 512K in the init script. Do this only for apq8084.

Change-Id: I2131c171d94b1c949b69d889506d6d97a1719bc6
Merge "rootdir: etc: change max readahead size to 512K in init"