Bug 1088683: Deal with GECKO_TOOLS_PREFIX environment variable.

TARGET_TOOLS_PREFIX can now be stored as GECKO_TOOLS_PREFIX in .var.profile
file [1]. This breaks profile-symbolicate.

profile-symbolicate now search for both TARGET_TOOLS_PREFIX and
GECKO_TOOLS_PREFIX.

[1] https://github.com/mozilla-b2g/gonk-misc/commit/b183966d7084f992ec5ff9ea6a486e20dd1fc6d6
Merge pull request #400 from walac/bugz/1088683

Bug 1088683: Deal with GECKO_TOOLS_PREFIX environment variable.