refactor(build): merge march32/64 directives

Both march32-directive and march64-directive eventually generate the
same march option that will passed to compiler.

Merge this two separate directives to a common one as march-directive.

Change-Id: I220d2b782eb3b54e13ffd5b6a581d0e6da68756a
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
diff --git a/docs/design/firmware-design.rst b/docs/design/firmware-design.rst
index 131cca1..3d648c4 100644
--- a/docs/design/firmware-design.rst
+++ b/docs/design/firmware-design.rst
@@ -2733,12 +2733,12 @@
 the toolchain  target architecture directive.
 
 Platform may choose to not define straight the toolchain target architecture
-directive by defining ``MARCH32_DIRECTIVE``.
+directive by defining ``MARCH_DIRECTIVE``.
 I.e:
 
 .. code:: make
 
-   MARCH32_DIRECTIVE := -mach=armv7-a
+   MARCH_DIRECTIVE := -mach=armv7-a
 
 Code Structure
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