fix(stm32mp2): build USB config in its own directory

As the compilation flags will change between SD-card configuration
and USB one, use a dedicated directory for USB build.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ic0877699503ac30270c1b3f1a8612d11a47826a1
diff --git a/script/tf-coverity/tf-cov-make b/script/tf-coverity/tf-cov-make
index bbfbed2..fd43937 100755
--- a/script/tf-coverity/tf-cov-make
+++ b/script/tf-coverity/tf-cov-make
@@ -695,7 +695,7 @@
 # STM32MP25 USB boot
 make ${stm32mp2_common_flags} STM32MP_USB_PROGRAMMER=1 \
     SPD=opteed STM32MP_DDR4_TYPE=1 \
-    BUILD_PLAT=build/stm32mp2-mp25-sdmmc/debug
+    BUILD_PLAT=build/stm32mp2-mp25-usb/debug
 
 # Platforms from TI
 make PLAT=k3 $(common_flags) all