The asm code to do the actual call into the kernel (or another
barebox) when compiled in thumb2 mode isn't necessary.  gcc generates
a perfectly good calling sequence from a normal function pointer call.
If it didn't, the code in bootstrap_boot() that uses a function
pointer to jump to the 2nd stage barebox from an xloader wouldn't
work.

It appears to be allowed that the call to kernel() could return, as
neither start_linux() nor kernel() are marked noreturn, and there is
code after calls to start_linux().

The asm code has a bug in this case, as it uses bx and not blx, and
thus doesn't set the link register.  Since it's a tail call, this
would be okay, but only if the LR value from the start of
start_linux() (and the callee-saved registers) are restored
beforehand, which isn't done.  The gcc generated call sequence will do
this.

Signed-off-by: Trent Piepho <tpiepho@kymetacorp.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>