Fix quote error from openthread/ot-reference-release#64

[lmnotran]

Fixes this error introduced by openthread/ot-reference-release#64

+ su -c 'INFRA_IF_NAME=eth0 RELEASE=1 REFERENCE_DEVICE=1 BACKBONE_ROUTER=1 NETWORK_MANAGER=0 DHCPV6_PD=0 WEB_GUI=0 REST_API=0 BORDER_ROUTING=0 NAT64=0 DNS64=0 OTBR_OPTIONS="-DOT_THREAD_VERSION=1.2 -DOTBR_DUA_ROUTING=ON -DOT_DUA=ON -DOT_MLR=ON -DOTBR_DNSSD_DISCOVERY_PROXY=OFF -DOTBR_SRP_ADVERTISING_PROXY=OFF -DOTBR_TREL=OFF -DOT_DIAGNOSTIC=ON -DOT_FULL_LOGS=ON -DOT_PACKAGE_VERSION=e6df00dd6 -DOTBR_PACKAGE_VERSION=bb565ca016 -DOT_POSIX_CONFIG_RCP_BUS=UART -DOTBR_RADIO_URL=spinel+hdlc+uart:///dev/ttyACM0?uart-baudrate=460800 -DOT_RCP_RESTORATION_MAX_COUNT=100" -DCMAKE_CXX_FLAGS='\''-DOPENTHREAD_CONFIG_MAC_CSL_REQUEST_AHEAD_US=5000'\'' script/bootstrap' pi
bash: -DCMAKE_CXX_FLAGS=-DOPENTHREAD_CONFIG_MAC_CSL_REQUEST_AHEAD_US=5000: command not found
+ cleanup
+ set +e
++ losetup -j /Users/matran/tmp/raspbian/raspbian.img --output NAME -n
+ loop_names=/dev/loop9
+ for loop in ${loop_names}
+ sudo umount -lf /dev/loop9p1
+ sudo umount -lf /dev/loop9p2
+ sudo losetup -d /dev/loop9
+ set -e

[lmnotran]

@jwhui can you review this please?