bcm21553-common: add support for device specific fstab

In order to use it, just add this declaration in your device tree's BoardConfig.mk
[*] TARGET_RECOVERY_FSTAB := /your/specific/recovery/fstab
The declaration written in the specific-device BoardConfig.mk will consider that declaration
more important, compared to the one in the common device tree, and will use that one.
Merge pull request #1 from streambinder/ics

bcm21553-common: add support for device specific fstab