Fix get_abs_build_var to work from below $ANDROID_BUILD_TOP
Fix get_abs_build_var to work from below $ANDROID_BUILD_TOP

I tried running gdbclient from a subdirectory of $ANDROID_BUILD_TOP
and found that the absolute paths generated by get_abs_build_var
included my subdirectory's path. This fixed get_abs_build_var to
changing directory in a subshell to $ANDROID_BUILD_TOP before invoking
make.

Change-Id: I12b9d9135ac7c044c21c88ed5705b22b0700c039

xorware/android_build,knone1/platform_manifest,xorware/android_build,RR-msm7x30/platform_manifest,xorware/android_build,see4ri/lolili,fallouttester/platform_manifest,xorware/android_build,xorware/android_build,pranav01/platform_manifest,ND-3500/platform_manifest,Hybrid-Power/startsync,hagar006/platform_manifest