HV: replace merge_cmdline api with strncat_s

Add a standard string api strncat_s() to replace merge_cmdline() to make code
more readable.

Another change is that the multiboot cmdline will be appended to the end of
configured SOS bootargs instead of the beginning, this would enable a feature
that some kernel cmdline paramter items could be overriden by multiboot cmdline
since the later one would win if same parameters configured in kernel cmdline.

Tracked-On: #4885

Signed-off-by: Victor Sun <victor.sun@intel.com>
Reviewed-by: Yonghua Huang <yonghua.huang@intel.com>
Reviewed-by: Yin Fengwei <fengwei.yin@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

hypervisor/include/arch/x86/seed.h

@@ -9,6 +9,7 @@
 
 #define BOOTLOADER_SEED_MAX_ENTRIES     10U
 #define BUP_MKHI_BOOTLOADER_SEED_LEN    64U
+#define MAX_SEED_ARG_SIZE		1024U
 
 /* Structure of seed info */
 struct seed_info {