ARM: zynq: Move BOOTM_NETBSD to Kconfig

The commit 4b0bcfa7 ("Kconfig: Migrate CONFIG_BOOTM_* options")
moved BOOTM_ options to Kconfig that's why align zynq defconfig with it.

Disabling NETBSD was done by commit d6f48ea5
("ARM: zynq: Do not enable NETBSD support by default").

Signed-off-by: Michal Simek <michal.simek@xilinx.com>