booting from qspi: update documentation to include buildroot ext support

Update the Booting from QSPI documentation to include support of nor.img
and nand.img images that can be created using the Microchip Buildroot
External.

Signed-off-by: Valentina Fernandez <valentina.fernandezalanis@microchip.com>

boards/mpfs-generic/booting-from-qspi/booting-from-qspi.md

@@ -31,9 +31,9 @@ The following flash memory devices are officially supported on PolarFire SoC:
 | Manufacturer  | Part No. | Type | Density       |
 |---------------|----------|------|---------------|
 | Winbond       | W25N01GV | NAND | 1Gb (128MB)   |
-| Micron*       | MT25QL256| NOR  | 256Mb (32MB)  |
+| Micron*       | MT25QL256| NOR | 256Mb (32MB)   |
 
-*Micron QSPI is only supported by the Meta PolarFire SoC Yocto BSP.
+*Micron QSPI is only supported in the Meta PolarFire SoC Yocto BSP and Microchip Buildroot External.
 
 Depending on the PolarFire SoC board being used the QSPI I/Os may be routed via the FPGA fabric to a Raspberry Pi interface, in this case an adapter daughter board may be required to connect a QSPI flash. See the individual sections below for more information.
 
@@ -106,9 +106,9 @@ The Micron MT25QL256 NOR flash memory can be connected to the Icicle Kit by usin
 
 ## Building a Linux image for a NAND or NOR flash memory device
 
-The [Microchip PolarFire SoC Yocto BSP](https://github.com/polarfire-soc/meta-polarfire-soc-yocto-bsp) layer allows building a minimal Linux image that can be programmed to the officially supported NAND and NOR flash memory devices.
+The [Microchip PolarFire SoC Yocto BSP](https://github.com/polarfire-soc/meta-polarfire-soc-yocto-bsp) layer and [Microchip Buildroot External](https://mi-v-ecosystem.github.io/redirects/repo-microchip-buildroot-external) allow users to build a minimal Linux image that can be programmed to the officially supported NAND and NOR flash memory devices.
 
-Linux images that are suitable for programming to a NAND or NOR memory device have a `.nand.mtdimg` or `.nor.mtdimg` file extension respectively. These images can be programmed to the QSPI flash memory using the HSS.
+Linux images created using the PolarFire SoC Yocto BSP layer and named per the `<image>-<machine>-<timestamp>.<flash-type>.mtdimg` format, for example, `core-image-minimal-mtdutils-icicle-kit-es-yyyyMMddHHmmSS.nand.mtdimg` or `core-image-minimal-mtdutils-icicle-kit-es-yyyyMMddHHmmSS.nor.mtdimg` are suitable for programming to a QSPI flash device using the HSS. For Microchip Buildroot External, images with the name `nand.img` or `nor.img` are used instead.
 
 <a name="programming-the-external-qspi-flash-memory-using-the-hss"></a>