CMake: add_custom_target does not require EXCLUDE_FROM_ALL

[mmicko]

add_custom_target syntax does not support EXCLUDE_FROM_ALL, actually it is reveresed, you need to add ALL if you wish to add it to all target.

Without this change you would get

make nextpnr-all-bba -j9

[ 20%] Generating chipdb-384.bba
[ 60%] Generating chipdb-1k.bba
[ 60%] Generating chipdb-8k.bba
[100%] Generating chipdb-u4k.bba
[100%] Generating chipdb-5k.bba
[100%] Built target nextpnr-ice40-bba
make[3]: EXCLUDE_FROM_ALL: No such file or directory
make[3]: *** [CMakeFiles/nextpnr-all-bba.dir/build.make:70: CMakeFiles/nextpnr-all-bba] Error 127
make[2]: *** [CMakeFiles/Makefile2:383: CMakeFiles/nextpnr-all-bba.dir/all] Error 2
make[1]: *** [CMakeFiles/Makefile2:390: CMakeFiles/nextpnr-all-bba.dir/rule] Error 2

[whitequark]

Thanks! Not sure how I managed to not catch it...