ARM: zynq: Move BOOTM_NETBSD to Kconfig

The commit 4b0bcfa7c4ec ("Kconfig: Migrate CONFIG_BOOTM_* options")
moved BOOTM_ options to Kconfig that's why align zynq defconfig with it.

Disabling NETBSD was done by commit d6f48ea5ce18
("ARM: zynq: Do not enable NETBSD support by default").

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/configs/zynq_virt_defconfig b/configs/zynq_virt_defconfig
index 58ce970..807e71f 100644
--- a/configs/zynq_virt_defconfig
+++ b/configs/zynq_virt_defconfig
@@ -16,6 +16,7 @@
 CONFIG_SPL_OS_BOOT=y
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SYS_SPI_U_BOOT_OFFS=0x100000
+# CONFIG_BOOTM_NETBSD is not set
 CONFIG_CMD_THOR_DOWNLOAD=y
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_DFU=y