.travis.yml: Refactor esp8266 build process.
.travis.yml: Refactor esp8266 build process.

To make it compatible with Github Actions. This includes avoiding defining
global variables referring to other variables, as Github doesn't expand
them (we actually still use "~" to refer to home dir, and need to run one
level of expansion explicitly).

Change-Id: Ib30517fc248d5e4b4147b06960563d6e6820dc82
Signed-off-by: Paul Sokolovsky <9dc5061f178bb11813e14f1ed02ba27eaf21fa5d@users.sourceforge.net>

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython