Extend Linux compat by adding kstrdup_const(), backed by lib/string.c.
This leverages U-Boots .rodata section on ARM64 to avoid pointlessly
duplicating const strings.
This is used by the Linux CCF_FULL port and may be useful elsewhere
in U-Boot.
Signed-off-by: Casey Connolly <casey.connolly@linaro.org>
139f5292e7