kernel: add support for limiting 4K erase sector support based on flash chip size

kernel: add support for limiting 4K erase sector support based on flash chip size

Some targets need 4K sectors for small flash chips (e.g. some
routerboards, where the entire chip is just one "erase block"), whereas
on other devices 4K sectors lead to horrible flash erase/write
performance.

Set the default limit in the generic kernel configuration to 4 MiB to
ensure that all new platforms don't use 4K sectors for bigger flash
chips. On all existing targets use 16 MiB for now to avoid regressions.
They will be changed individually in follow-up commits.

Signed-off-by: Felix Fietkau <nbd@nbd.name>