boot_serial: Allow using a buffer larger than 512 bytes

There are 3 levels of buffers and only the first one seems to be
configurable, this fixes that issue.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig.serial_recovery b/boot/zephyr/Kconfig.serial_recovery
index e254ede..225c430 100644
--- a/boot/zephyr/Kconfig.serial_recovery
+++ b/boot/zephyr/Kconfig.serial_recovery
@@ -52,10 +52,16 @@
 	  selection is done.
 
 config BOOT_MAX_LINE_INPUT_LEN
-	int "Maximum command line length"
+	int "Maximum input line length"
 	default 512
 	help
-	  Maximum length of commands transported over the serial port.
+	  Maximum length of input serial port buffer.
+
+config BOOT_SERIAL_MAX_RECEIVE_SIZE
+	int "Maximum command line length"
+	default 1024
+	help
+	  Maximum length of received commands via the serial port.
 
 config BOOT_SERIAL_DETECT_PORT
 	string "GPIO device to trigger serial recovery mode (DEPRECATED)"