We use BOOTSOURCE_SPI to denote, among other things, QSPI on i.MX7 and
VFxxx, whereas on i.MX6 it is used to mean SPI-NOR. To make functions
like imx_xload() work consistently across various i.MX platforms use
already existent BOOTSOURCE_SPI_NOR constant to mean booting from
SPI-NOR on i.MX6 as well.

Replace all in-tree code that relying on the old value as well.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>