boards: sam_e70_xplained: set GPNVM1 bit when flashing

By default a factory new SAM E70 chip will boot SAM-BA boot loader located in
the ROM, not the flashed image. This is determined by the value of GPNVM1
(General-Purpose NVM bit 1). Updated flash procedure will ensure that GPNVM1
is set to 1 changing the default behavior to boot from Flash.

Change-Id: Ic6334c9d4743a7665fc944e8f49fc1467ecda40d
Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>