arm: Fix _start for CONFIG_SYS_DV_NOR_BOOT_CFG

arm: Fix _start for CONFIG_SYS_DV_NOR_BOOT_CFG

The boards using CONFIG_SYS_DV_NOR_BOOT_CFG (i.e. calimain,
da850evm_direct_nor and enbw_cmc) had the _start symbol defined after
the CONFIG_SYS_DV_NOR_BOOT_CFG word rather than before it in
arch/arm/lib/vectors.S. Because of that, if by lack of luck
'gd->mon_len = (ulong)&__bss_end - (ulong)_start' (see setup_mon_len())
was a multiple of 4 kiB (see reserve_uboot()), then the last BSS word
overlapped the first word of the following reserved RAM area (or went
beyond the top of RAM without such an area) after relocation because
__image_copy_start did not match _start (see relocate_code()).

This was broken by commit 41623c9 'arm: move exception handling out of
start.S files', which defined _start twice (before and after the
CONFIG_SYS_DV_NOR_BOOT_CFG word), then by commit 0a26e1d 'arm: fix a
double-definition error of _start symbol', which kept the definition of
the _start symbol after the CONFIG_SYS_DV_NOR_BOOT_CFG word. This new
commit fixes this issue by restoring the original behavior, i.e. by
defining the _start symbol before the CONFIG_SYS_DV_NOR_BOOT_CFG word.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau.dev@gmail.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Manfred Rudigier <manfred.rudigier@omicron.at>
Cc: Christian Riesch <christian.riesch@omicron.at>
Cc: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Cc: Heiko Schocher <hs@denx.de>