PICO_W support and some small improvements

[peterpar]

• added 2 BDOS dummy functions: attribute and flush
• added support for PICO_W board in CMakeLists.txt and in code
• added verification calculations to catch potential issues with flash disks being declared over code
• determined safe flash disk start address for PICO_W mode
• removed duplicating BlockDeviceParams declaration from header, replaced with extern and declared in C file
• allow some fcb errors to continue for diagnostic purposes

[kevinboone]

Thank you for all this work. Much appreciated.