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
1 file changed