Clamp FLASH size to 8K to match Hubris

[mkeeter]

The hash of target/thumbv8m.main-none-eabihf/release/bootleby doesn't change, so this should be innocuous (but prevents us from being Unpleasantly Surprised further down the line)

[mkeeter]

Apparently Linux has different opinions on the resulting binary size versus MacOS (where a debug build works fine) 🫠

Here's the debug build size on my machine, which just barely squeezes under the line:

arm-none-eabi-size target/thumbv8m.main-none-eabihf/debug/bootleby
   text    data     bss     dec     hex filename
   7616       0  721988  729604   b2204 target/thumbv8m.main-none-eabihf/debug/bootleby