esp8266: Fix broken garbage collector

[atx]

As user_init() is not a true main functions, the stack pointer captured within
is not pointing at the base of the stack. This caused gc_collect being called
with sp being higher than stack_end, causing integer overflow and crashing as
gc tried to scan almost the entire address space.

[pfalcon]

This doesn't look like a "fix", but rather as a workaround. Then, it's worth an inline comment why STACK_END is used like that, because info in commit message is not so easily discoverable.

[coveralls]

Coverage increased (+0.01%) to 92.85% when pulling 4b85b98 on atalax:esp-fixgc into cd87d20 on micropython:master.

[atx]

Then, it's worth an inline comment why

Added a comment.

[dpgeorge]

Merged in 28076f3.

[coveralls]

Coverage remained the same at 92.85% when pulling 9fda441 on atalax:esp-fixgc into 9a42eb5 on micropython:master.