Amends#8507I took the liberty to also do some refactoring; specifically, fixing signed vs. unsigned mismatch in len, using pointer object vs. the original manual malloc & free, try to have named constants for certain addresses and lengths, plus localize printing of u8 arrays.The suggested test to have a 'dummy' verifier works just fine. (...how it actually works and gets the hash to compare with is a whole other question, though)Another issue noticed while testing, in the underlying bearssl api there's an actual limit for hash length.https://github.com/earlephilhower/bearssl-esp8266/blob/6105635531027f5b298aa656d44be2289b2d434f/inc/bearssl_rsa.h#L257

…ature appended to the image (esp8266#8507)The _hash and _verify functionality of the Updater class are pretty much entwined. But it might be useful to calculate the hash, even without a signature present in the image itself. This change permits that by allowing a zero-length signature.For reference, one possible use case is where the expected hash is provided separately to the uploaded image. Another is to provide generic post-upload validation of the image: the hash function permits a convenient way of inspecting the complete image against arbitrary conditions; this is particularly useful after HTTP client OTA updates.(It's fair that reading the whole image back out of flash is not very efficient, but that's not the concern of this PR.)

Amendsesp8266#8507I took the liberty to also do some refactoring; specifically, fixing signed vs. unsigned mismatch in len, using pointer object vs. the original manual malloc & free, try to have named constants for certain addresses and lengths, plus localize printing of u8 arrays.The suggested test to have a 'dummy' verifier works just fine. (...how it actually works and gets the hash to compare with is a whole other question, though)Another issue noticed while testing, in the underlying bearssl api there's an actual limit for hash length.https://github.com/earlephilhower/bearssl-esp8266/blob/6105635531027f5b298aa656d44be2289b2d434f/inc/bearssl_rsa.h#L257