bsps/stm32h7: link to flash on STM32H7B3I-DK

The patch is needed due to smaller SRAM and completely disabled
SDRAM on the BSP variant.

spec/build/bsps/arm/stm32h7/optlinkcmds.yml

@@ -3,7 +3,10 @@ actions:
 - env-assign: null
 build-type: option
 default: linkcmds.sdram
-default-by-variant: []
+default-by-variant:
+- value: linkcmds.flash
+  variants:
+  - arm/stm32h7b3i-dk
 enabled-by: true
 format: '{}'
 links: []