Build - envsetup: push to connected device only if it is $CM_BUILD

Change-Id: I0cd9b4c0af2f91c62190eddf13263f3a0530f5b4
Signed-off-by: STELIX <ssspinni@gmail.com>
Build - envsetup: Don't tread on my $PROMPT_COMMAND

* I want my own PROMPT_COMMAND to coexist with Android's, so stash it
   in the environment and add it to the new command.

envsetup: Fix $PROMPT_COMMAND

Change I200902f135d0c5c620a8eb3ce9cefdba318cf9d3 broke the android
PROMPT_COMMAND on scenarios where the existing prompt already had
a hardstatus adjustment, since appending it to Android's just clobbered
Android's in favor of the original.

So instead of appending them, inject android's at the beggining of the
existing one (and create a default if none exists)

envsetup: Really fix PROMPT_COMMAND handling

* Add ANDROID_NO_PROMPT_COMMAND to not change it at all.
* Export ANDROID_PROMPT_PREFIX so it can be used elsewhere.

envsetup: More PROMPT_COMMAND

* Get rid of ANDROID_NO_PROMPT_COMMAND, the pre-existing
STAY_OFF_MY_LAWN
does the same thing
* Remove any pre-existing android prefixes to avoid accumulation
scenarios in consecutive builds (i.e., multiple build identification
strings in hardstatus)

envsetup: add quotes around PROMPT_COMMAND

Better compatibility for zsh.

Change-Id: I05fe82298ef75a4d38ccc6587eda772357806865
Signed-off-by: STELIX <ssspinni@gmail.com>