buildgcc: Remove all bashisms, making the script run also on BSD

buildgcc: Remove all bashisms, making the script run also on BSD

Use sed instead of ${variable:start:length} and ${#variable}
Use single = in string comparisons
Use `eval echo '$'$variable` instead of ${!variable}
Use > file 2>&1 instead of &> file
Use readlink -f to expand the path of GCC configure

Change-Id: Idc7dfcea3922f55630a6855acdb19e36582708bd
Signed-off-by: Peter Stuge <peter@stuge.se>
Reviewed-on: http://review.coreboot.org/165
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Tested-by: build bot (Jenkins)