Allow to specify maximum ROM size

[fooker]

The new mechanism to generate linker scripts does not allow to specify a maximum ROM size.

I used to limit the irom0_0_seg length to (1M / 2 - 0x2010) which allows to ensure that the first rom will now overwrite the second one. For me, both ROMs should have the same 0x2010 offset from the base addresses (0x00000 for ROM 0 and 0x80000 for ROM 1). The remaining space between the two ROMs is used to store some persistent data.

It would be nice to have a way to specify this limit again. Or as an alternative, overwrite the generative linker scripts with provided ones.

[fooker]

See #2044 for a possible solution.

[slaff]

With the arrival of (partition tables)[https://sming.readthedocs.io/en/latest/_inc/Sming/Components/Storage/index.html] this should be possible. Feel free to re-open this ticket if you have problems achieving what you want with the latest develop version.