This effectively reverts the change that introduced this new constant.
The reason is so that users do not need to rebuild the filesystem on
their modules when upgrading the firmware.
Users can change RESERVED_SECS by hand if they need the feature, and in
future firmware it may default to a non-zero value.
This code is no longer pertinent for some time - since switchover to
SDK2.0, there must be correct flash size set for bootloader, or there's
a risk of flash data corruption. And indeed, the correct flash size is
by default auto-detected by esptool.py 1.2.
Starting at esp.flash_user_start(), the reserved sectors are for general
purpose use, for example for native code generation. There is currently
one sector reserved as such.
As we're looking towards adding OTA support, calculation of a FlashROM
area which can be used for filesystem (etc.) may become complex, so
introduce C function for that. So far it just hardcodes current value,
0x90000. In the future the function may be extended (and renamed) to
return the size of area too.
To start with, the critical scripts _boot.py and flashbdev.py are frozen
to improve performance and reduce RAM consumption.
Saves about 1000 bytes of heap RAM for a bare boot with filesystem.
