boot: arm: Enable support for custom board_prep_linux

Once the arch specific boot_prepare_linux completes, boards wants to
have a custom preparation for linux. Add support for a custom
board_prep_linux.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
diff --git a/arch/arm/lib/bootm.c b/arch/arm/lib/bootm.c
index 1638f1e..488358a 100644
--- a/arch/arm/lib/bootm.c
+++ b/arch/arm/lib/bootm.c
@@ -224,6 +224,8 @@
 }
 #endif
 
+__weak void board_prep_linux(bootm_headers_t *images) { }
+
 /* Subcommand: PREP */
 static void boot_prep_linux(bootm_headers_t *images)
 {
@@ -270,6 +272,8 @@
 		printf("FDT and ATAGS support not compiled in - hanging\n");
 		hang();
 	}
+
+	board_prep_linux(images);
 }
 
 __weak bool armv7_boot_nonsec_default(void)