KM/IVM: split the IVM reading and parsing in 2 parts

This allows to first read the IVM content (earlier in the boot sequence)
and define the ethaddr env variable thanks to the ivm_read_eepromi().
Later, the IVM content can be parsed and used to define some hush
variables, when the hush subsystem is available thanks to
ivm_analyze_eeprom().

To avoid the HW read to happen twice, the buffer passed to
ivm_read_eeprom() has to be reused by ivm_analyze_eeprom (and thus
allocated before calling ivm_read_eeprom()).

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>