pdk: fix build error on mac

About symlink, show error on Mac.
To fix it, use 'rm' before 'cp'.

error log:
cp: symlink: /data/misc/audio/wcd9310_anc.bin: File exists
make: *** [out/target/product/mako/system/etc/firmware/wcd9310/wcd9310_anc.bin] Error 1

Bug: 8127112

Change-Id: I94e1677a6742cf4350e92bdffd58e6b782cf662c