avb2.0: add proper dependencies

1. Since libavb library alone is highly portable, introduce dedicated
Kconfig symbol for AVB bootloader-dependent operations, so it's possible
to build libavb separately. AVB bootloader-dependent operations include:
  * Helpers to process strings in order to build OS bootargs.
  * Helpers to access MMC, similar to drivers/fastboot/fb_mmc.c.
  * Helpers to alloc/init/free avb ops.
2. Add CONFIG_FASTBOOT dependency, as fastboot buffer is
re-used in partition verification operations.

Reported-by: Eugeniu Rosca <rosca.eugeniu@gmail.com>
Signed-off-by: Igor Opaniuk <igor.opaniuk@linaro.org>
Reviewed-by: Eugeniu Rosca <rosca.eugeniu@gmail.com>
Tested-by: Eugeniu Rosca <rosca.eugeniu@gmail.com>
diff --git a/common/Makefile b/common/Makefile
index 66584f8..7100541 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -121,4 +121,4 @@
 obj-y += s_record.o
 obj-y += xyzModem.o
 
-obj-$(CONFIG_LIBAVB) += avb_verify.o
+obj-$(CONFIG_AVB_VERIFY) += avb_verify.o