lib: consolidate hang()

Delete all occurrences of hang() and provide a generic function.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Acked-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
[trini: Modify check around puts() in hang.c slightly]
Signed-off-by: Tom Rini <trini@ti.com>