Use of define _XUA_ENABLE_BOS_DESC not consistently handled

[humphrey-xmos]

_XUA_ENABLE_BOS_DESC defined in xua_ep0_descriptors.h is used in many other header files.

xua_ep0_descriptors.h contains data definitions and is a header that cannot be included in more than one source file (xua_endpoint0.c).

The use of _XUA_ENABLE_BOS_DESC across more than one header works by luck rather than design.

Suggestions:

1. define _XUA_ENABLE_BOS_DESC in a separate header file, other than xua_ep0_descriptors.h,
2. Or, remove data definitions from header files.
3. Other...