tools: usb: ffs-test: Fix build on big endian systems

The tools/usb/ffs-test.c file defines cpu_to_le16/32 by using the C
library htole16/32 function calls. However, cpu_to_le16/32 are used when
initializing structures, i.e in a context where a function call is not
allowed.

It works fine on little endian systems because htole16/32 are defined by
the C library as no-ops. But on big-endian systems, they are actually
doing something, which might involve calling a function, causing build
failures, such as:

   ffs-test.c:48:25: error: initializer element is not constant
    #define cpu_to_le32(x)  htole32(x)
                            ^~~~~~~
   ffs-test.c:128:12: note: in expansion of macro ‘cpu_to_le32’
      .magic = cpu_to_le32(FUNCTIONFS_DESCRIPTORS_MAGIC_V2),
               ^~~~~~~~~~~

To solve this, we code cpu_to_le16/32 in a way that allows them to be
used when initializing structures. This fix was imported from
meta-openembedded/android-tools/fix-big-endian-build.patch written by
Thomas Petazzoni <thomas.petazzoni@free-electrons.com>.

CC: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Senna Tschudin <peter.senna@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

tools/usb/ffs-test.c

@@ -44,12 +44,25 @@
 
 /******************** Little Endian Handling ********************************/
 
-#define cpu_to_le16(x)  htole16(x)
-#define cpu_to_le32(x)  htole32(x)
+/*
+ * cpu_to_le16/32 are used when initializing structures, a context where a
+ * function call is not allowed. To solve this, we code cpu_to_le16/32 in a way
+ * that allows them to be used when initializing structures.
+ */
+
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+#define cpu_to_le16(x)  (x)
+#define cpu_to_le32(x)  (x)
+#else
+#define cpu_to_le16(x)  ((((x) >> 8) & 0xffu) | (((x) & 0xffu) << 8))
+#define cpu_to_le32(x)  \
+	((((x) & 0xff000000u) >> 24) | (((x) & 0x00ff0000u) >>  8) | \
+	(((x) & 0x0000ff00u) <<  8) | (((x) & 0x000000ffu) << 24))
+#endif
+
 #define le32_to_cpu(x)  le32toh(x)
 #define le16_to_cpu(x)  le16toh(x)
 
-
 /******************** Messages and Errors ***********************************/
 
 static const char argv0[] = "ffs-test";