[build] Use PRODUCT_SHORT_NAME for end-user visible strings

[build] Use PRODUCT_SHORT_NAME for end-user visible strings

Use PRODUCT_SHORT_NAME instead of a hardcoded "iPXE" for strings which
are typically shown in the user interface.

Note that this only allows for customisation of the user interface.
Where the "iPXE" string serves a technical purpose (such as in the
HTTP User-Agent), the string cannot be customised.

Signed-off-by: Michael Brown <mcb30@ipxe.org>