Makefile could clarify use of TARGET variable

[stevekerrison]

Build system will use an XN or target system name. It's not clear, however, that when specifying an XN file one should leave out the extension. This might be documented elsewhere but it's probably worth mentioning in its usage description, e.g. in https://github.com/xcore/xcore_template/blob/master/app_template/Makefile#L1

Hmmm. Maybe the makefiles should just handle the case where the user puts .xn after it. This should be pretty safe (people who put both a.xn and a.xn.xn in there repository deserve everything they get)

[stevekerrison]

Good idea.