helpers/envkernel.sh: add gawk when initialising chroot
The Awk implementation from BusyBox cannot run the arch/arm64/tools/gen-sysreg.awk script, and results in the following error:
GEN arch/arm64/include/generated/asm/sysreg-defs.h
Error at 51: unhandled statement
make[2]: *** [../arch/arm64/tools/Makefile:24: arch/arm64/include/generated/asm/sysreg-defs.h] Error 1
make[2]: *** Deleting file 'arch/arm64/include/generated/asm/sysreg-defs.h'
make[1]: *** [../arch/arm64/Makefile:176: archprepare] Error 2
make[1]: Leaving directory '/mnt/linux/.output'
make: *** [Makefile:228: __sub-make] Error 2
make: Leaving directory '/mnt/linux'
Add gawk (GNU Awk) to work around this issue.