bitbake: ast: Add error when trying to use dash in sh function names

bitbake: ast: Add error when trying to use dash in sh function names

A dash character is illegal in function names in sh (but not bash). Since
our shell tasks run under sh and the shell parser is sh based, EXPORT_FUNCTIONS
won't work with class names containing a dash.

We can't change sh, we can ensure the user is warned about the problem
straight away though.

[YOCTO #7006]

(Bitbake rev: 879fe20f47ba75f4afb3484d4398d5fd60431e12)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>