prepare_env.sh: fix some shellcheck warnings

prepare_env.sh: fix some shellcheck warnings

prepare_env.sh:13:3: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:14:3: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:15:3: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:16:3: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:17:3: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:18:3: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:22:8: warning: Declare and assign separately to avoid masking return values. [SC2155]
prepare_env.sh:28:1: warning: INSTALLERS appears unused. Verify it or export it. [SC2034]
prepare_env.sh:36:5: warning: Can't follow non-constant source. Use a directive to specify location. [SC1090]
prepare_env.sh:42:1: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:46:5: warning: Can't follow non-constant source. Use a directive to specify location. [SC1090]
prepare_env.sh:46:12: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:53:5: warning: i appears unused. Verify it or export it. [SC2034]
prepare_env.sh:54:27: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:68:25: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:73:19: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:73:49: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:76:27: note: Useless cat. Consider 'cmd < file | ..' or 'cmd file | ..' instead. [SC2002]
prepare_env.sh:77:24: note: Useless cat. Consider 'cmd < file | ..' or 'cmd file | ..' instead. [SC2002]
prepare_env.sh:80:46: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:83:46: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:86:46: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:89:43: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:92:43: note: Double quote to prevent globbing and word splitting. [SC2086]
prepare_env.sh:96:71: note: Double quote to prevent globbing and word splitting. [SC2086]

Change-Id: I96a1f591b825d21e3ba202f1d681449347d940a6
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>