arm/cpu: Avoid multiple definitions of barebox_arm_entry
All versions of barebox_arm_entry (in uncompress.c, start.c and
start-pbl.c) appear to be doing exacty the same thing. So move the
definition into a separate file and use IS_ENABLED macro to avoid
re-definition.
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>