avb: Make AVB independent of fastboot

avb: Make AVB independent of fastboot

AVB only uses CONFIG_FASTBOOT_BUF_ADDR from fastboot for memory.
This memory is used for assigning temporary buffers.
This can be assigned a new variable and used as CONFIG_AVB_BUF_ADDR.
This is to support future boards that support AVB but dont support
USB and therefore dont support FASTBOOT.

Signed-off-by: Usama Arif <usama.arif@arm.com>
Cc: Igor Opaniuk <igor.opaniuk@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Igor Opaniuk <igor.opaniuk@gmail.com>
[trini: Change defaults]
Signed-off-by: Tom Rini <trini@konsulko.com>