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/Kconfig b/common/Kconfig
index dc2e1c2..9f6a162 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -637,6 +637,16 @@
 	  and the algorithms it supports are defined in common/hash.c. See
 	  also CMD_HASH for command-line access.
 
+config AVB_VERIFY
+	bool "Build Android Verified Boot operations"
+	depends on LIBAVB && FASTBOOT
+	help
+	  This option enables compilation of bootloader-dependent operations,
+	  used by Android Verified Boot 2.0 library (libavb). Includes:
+	    * 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.
+
 endmenu
 
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