board: stm32mp1: set environment variable fdtfile

For booting Linux in the generic distro mechanism and support of
FDTDIR in extlinux.conf , cmd/pxe.c retrieves the FDT file name
from "fdtfile" environment variable.

Dynamically build this variable with compatible of STMicroelectronics
boards.
Signed-off-by: NPatrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: NPatrice Chotard <patrice.chotard@st.com>

board/st/stm32mp1/stm32mp1.c

@@ -699,10 +699,19 @@ int board_late_init(void)
 	fdt_compat = fdt_getprop(gd->fdt_blob, 0, "compatible",
 				 &fdt_compat_len);
 	if (fdt_compat && fdt_compat_len) {
-		if (strncmp(fdt_compat, "st,", 3) != 0)
+		if (strncmp(fdt_compat, "st,", 3) != 0) {
 			env_set("board_name", fdt_compat);
-		else
+		} else {
+			char dtb_name[256];
+			int buf_len = sizeof(dtb_name);
+
 			env_set("board_name", fdt_compat + 3);
+
+			strncpy(dtb_name, fdt_compat + 3, buf_len);
+			buf_len -= strlen(fdt_compat + 3);
+			strncat(dtb_name, ".dtb", buf_len);
+			env_set("fdtfile", dtb_name);
+		}
 	}
 	ret = uclass_get_device_by_driver(UCLASS_MISC,
 					  DM_GET_DRIVER(stm32mp_bsec),