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/doc/README.avb2 b/doc/README.avb2
index 67784b5..120279f 100644
--- a/doc/README.avb2
+++ b/doc/README.avb2
@@ -58,6 +58,7 @@
 -----------------------------------
 The following options must be enabled:
 CONFIG_LIBAVB=y
+CONFIG_AVB_VERIFY=y
 CONFIG_CMD_AVB=y