Document that `PARTITION_RESERVED_SIZE` values are configurable using environment variables

[petefoth]

See https://source.android.com/docs/core/ota/dynamic_partitions/implement#dyanimic-partitions-sizing

To fix #616 I had to tinker with the BOARD_EXTIMAGE_PARTITION_RESERVED_SIZE value

A while ago there was discussion about the size of the size of the system image, and how this is affected by the WITH_GMSflag

These variables are conditionally set in vendor/lineage/config/BoardConfigReservedSize.mk . they are set using the ?= make operator to set the variable only if it is not already set (see this blog post). So the values in this file are

• conditional on the values of WITH_GMS
• are only used if the variable is not already set (.e.g. in the build environment or in device-specific makefiles)

So, if we give these environment variables a value (using a -e statement in the docker run command), then this value will overrride the value in BoardConfigReservedSize.mk, but will themselves be overridden by any values set in device-specific makefiles.

At the moment, we need to give BOARD_EXTIMAGE_PARTITION_RESERVED_SIZE a bigger value, otherwise our 21.0 builds will fail, at least for some devices. We don't need to mess with the values of the other PARTITION_RESERVED_SIZE variables, but we should document in the README.md that if builders do need to change the value of those variables, they can do so by setting them in a using a -e statement in the docker run command. We don't need to implement our own variable for this. This applies for all the variables that are conditionally set in BoardConfigReservedSize.mk` including:

BOARD_PRODUCTIMAGE_PARTITION_RESERVED_SIZE
BOARD_SYSTEMIMAGE_PARTITION_RESERVED_SIZE
BOARD_SYSTEM_EXTIMAGE_PARTITION_RESERVED_SIZE