ARMv7-R and TMS570: Re-orider some initialization logic. __start used to...

ARMv7-R and TMS570: Re-orider some initialization logic.  __start used to called arm_boot() which would return.  Then __start would call os_start().  That won't work for the TMS570 if is does a destructive memory tested because the return information will be lost in the stack.  Also comment a nuisance assertion. The assertion is probably correct but certainly a nuisance during initial testing