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/cmd/Kconfig b/cmd/Kconfig
index c5a7a3c..0cf530d 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1779,7 +1779,7 @@
 
 config CMD_AVB
 	bool "avb - Android Verified Boot 2.0 operations"
-	depends on LIBAVB
+	depends on AVB_VERIFY
 	default n
 	help
 	  Enables a "avb" command to perform verification of partitions using