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To simplify the rule that creates the concatenated binary, use ASFLAGS
instead of adding all paths in the AS command line. This allows a better
management if a binary is not present.

Change-Id: Ic8b4566e7dedc6f55be355a92e3b214cef138d9b
Signed-off-by: Yann Gautier <yann.gautier@st.com>