Makefile: pass -undef option to cmd_gen_envp

Without the '-undef' option, the 'linux' string in .env files is
replaced with the string '1 '.
For example, in the board/armadeus/opos6uldev/opos6uldev.env file,

kernelimg=opos6ul-linux.bin

becomes

kernelimg=opos6ul-1 .bin

in the include/generated/env.in file.

That's because 'linux' is a System-specific Predefined Macros. [1]

Pass the '-undef' option to fix this issue.

[1] https://gcc.gnu.org/onlinedocs/gcc-13.2.0/cpp/System-specific-Predefined-Macros.html



Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Reviewed-by: Tom Rini <trini@konsulko.com>