Docs lie about default of riscv BSP_FDT_BLOB_SIZE_MAX

[Jens Schweikhardt]

Chasing the reason for why a tiny RTEMS app is about 300KB in size, I found that the default value as displayed by

$ ./waf --rtems-bsps=riscv/rv32imafc bspdefaults|grep BLOB_SIZE
BSP_FDT_BLOB_SIZE_MAX = 262144

does not match the one stated in ​https://docs.rtems.org/branches/master/user/bsps/bsps-riscv.html#build-configuration-options which says 65536.

Desired action: make code and docs agree on a default value.