marvell: Move BLE from external repo to the platform folder

The BLE is the pre-TF-A boot stage required by Marvell Armada
BootROM for bringing up DRAM and allow the boot image copy to it.
Since this is not a standard boot level and only uses the TF-A
as a build environment, it was introduced out of source tree.
However it turns out that such remote location introduces additional
complexity to the upstream TF-A build process.
In order to simplify the build environment the BLE source folder
is relocated from the external repository to A8K platform directory.
The build documentation is updated accordingly.

Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
diff --git a/plat/marvell/a8k/common/a8k_common.mk b/plat/marvell/a8k/common/a8k_common.mk
index 364935c..49745ce 100644
--- a/plat/marvell/a8k/common/a8k_common.mk
+++ b/plat/marvell/a8k/common/a8k_common.mk
@@ -106,7 +106,7 @@
 include $(PLAT_COMMON_BASE)/mss/mss_a8k.mk
 
 # BLE (ROM context execution code, AKA binary extension)
-BLE_PATH	?=  ble
+BLE_PATH	?=  $(PLAT_COMMON_BASE)/ble
 
 include ${BLE_PATH}/ble.mk
 $(eval $(call MAKE_BL,e))