gcc 10 no longer lets you get away with not externing global variables in header files. This adds the necessary extern and adds defines in appropriate c files
By using `BOOT_CFG_AUTOBOOT_EN` in `boot_cfg` and `EXTRA_CFG_NYX_UMS` in `extra_cfg` you can launch UMS from boot.
The `ums` variable must be populated with one of the following:
```
NYX_UMS_SD_CARD 0
NYX_UMS_EMMC_BOOT0 1
NYX_UMS_EMMC_BOOT1 2
NYX_UMS_EMMC_GPP 3
NYX_UMS_EMUMMC_BOOT0 4
NYX_UMS_EMUMMC_BOOT1 5
NYX_UMS_EMUMMC_GPP 6
```
