build: Merge default-by-family into by-variant

Prefix the BSP family name with "bsps/" to make it distinct to the BSP
variant names.

Update #4468.

spec/build/bsps/lm32/lm32_evr/abi.yml

@@ -7,7 +7,6 @@ build-type: option
 copyrights:
 - Copyright (C) 2020 embedded brains GmbH (http://www.embedded-brains.de)
 default: []
-default-by-family: []
 default-by-variant: []
 description: |
   ABI flags

spec/build/bsps/lm32/lm32_evr/optsim.yml

@@ -6,7 +6,6 @@ build-type: option
 copyrights:
 - Copyright (C) 2020 embedded brains GmbH (http://www.embedded-brains.de)
 default: false
-default-by-family: []
 default-by-variant:
 - value: true
   variants: