Add a build directory for libbearssl.a

[earlephilhower]

Simple git submodule and makefile for building the bearssl library
from source in the Arduino tree.

[earlephilhower]

@d-a-v , I tried to copy your setup in lwip2 for this. Can you eyeball things and see if I've got it right? Thanks!

[d-a-v]

That works for me !
libssl.a is regenerated with make install
What do you think of adding a make merge-upstream or so to manually add the remote upstream and do the git pull (for test purpose only) ?

[devyte]

LGTM!!

[earlephilhower]

Several fixes/updates per the gitter discussion:

• Fixed the makefile install to also copy over the headers
• Add a git remote to upstream on submodule init, and a merge-upstream makefile target
• Generate a git hash ID on install, and include the hash in the ESP.getFullVersion() call