vexpress64: Kconfig: move board definitions out of arch/arm

At the moment we define three "VExpress64" boards in arch/arm/Kconfig,
plus have a second Kconfig file in board/armltd/Kconfig.
One of those three boards is actually bogus (TARGET_VEXPRESS64_AEMV8A),
that stanza looks like being forgotten in a previous cleanup.

To remove the clutter from the generic Kconfig file, just define some
ARCH_VEXPRESS64 symbol there, enable some common options, and do the
board/model specific configuration in the board/armltd Kconfig file.

That allows to streamline and fine tune the configuration later, and
to also pull a lot of "non user choices" out of the defconfigs.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/board/armltd/vexpress64/Kconfig b/board/armltd/vexpress64/Kconfig
index 4aab3f0..55fe011 100644
--- a/board/armltd/vexpress64/Kconfig
+++ b/board/armltd/vexpress64/Kconfig
@@ -1,4 +1,4 @@
-if TARGET_VEXPRESS64_BASE_FVP || TARGET_VEXPRESS64_JUNO
+if ARCH_VEXPRESS64
 
 config SYS_BOARD
 	default "vexpress64"
@@ -9,6 +9,24 @@
 config SYS_CONFIG_NAME
 	default "vexpress_aemv8"
 
+choice
+	prompt "VExpress64 board variant"
+
+config TARGET_VEXPRESS64_BASE_FVP
+	bool "Support Versatile Express ARMv8a FVP BASE model"
+	select SEMIHOSTING
+
+config TARGET_VEXPRESS64_JUNO
+	bool "Support Versatile Express Juno Development Platform"
+	select DM_ETH
+	select USB
+	select OF_CONTROL
+	select CLK
+	select BLK
+	imply OF_HAS_PRIOR_STAGE
+
+endchoice
+
 config JUNO_DTB_PART
 	string "NOR flash partition holding DTB"
 	default "board.dtb"