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>

cmd/Kconfig

@@ -1779,7 +1779,7 @@ config CMD_TRACE
 
 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

common/Kconfig

@@ -637,6 +637,16 @@ config HASH
 	  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
 
 menu "Update support"

common/Makefile

@@ -121,4 +121,4 @@ obj-$(CONFIG_$(SPL_)LOG_CONSOLE) += log_console.o
 obj-y += s_record.o
 obj-y += xyzModem.o
 
-obj-$(CONFIG_LIBAVB) += avb_verify.o
+obj-$(CONFIG_AVB_VERIFY) += avb_verify.o

doc/README.avb2

@@ -58,6 +58,7 @@ Slot verification result: ERROR_IO
 -----------------------------------
 The following options must be enabled:
 CONFIG_LIBAVB=y
+CONFIG_AVB_VERIFY=y
 CONFIG_CMD_AVB=y