The partition table of the device in the esp8266 and esp32 based boards
was set to a default table with one "factory" partition with exactly
the size of the compiled firmware. This is problematic if we want to
update the device on the field.
This patch allows to set the `PARTITION_TABLE_CSV` variable from the
Makefile to a .csv file with a custom partition table, for example this
could be set to a partition table with two ota entries, or with a single
factory entry but of a known fixed size.
