build: use BOARD_CUSTOM_BOOTIMG_MK to clean recovery (5MB max)

Remove /sbin/cbd which is copied from boot.img ramdisk to fit
the 5242880 bytes maximum.

On the Galaxy R, you can flash the recovery in download mode :
 heimdall flash --SOS out/target/product/i9103/recovery.img

Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>

Change-Id: Ib8fca27197c3f58fd1df8138aeff12360f7f417d